I've been good friends with a guy for about 18months. We are both single and in our early twenties. I started fancying him pretty much immediately but after a series of friendships going sour from telling guys I liked them, I decided to give it time to see if things developed.

Friendships do tend to go sour with the opposite sex, if they find out you fancy them.  For some reason people feel the need to avoid someone that fancies them, almost as if it becomes a difficult situation that they don't know how to react to.

We've become closer and talked about past relationships, future hopes, our sexual history (we're both virgins) etc. I was feeling insecure about my potential for a relationship and he said that I had everything going for me and had a better body than his ex etc.

There are loads of women that I think have great bodies but I would never go out with them because their personality doesn't match what I want from a woman and a long term relationship.  But it seems he is trying to butter you up for some reason here.  He likes the look of your body and that might be a clue that he wants to get intimate with your body.

I've found out since that he knows I fancy him, so why would he say that unless he's trying to lead me on?

Most men don't find out that women fancy them.  They never trust gossip and they never rely too much on their gut instinct about whether a woman fancies them (because they are usually wrong).  So in essence I know your saying that he thinks you fancy him, but in reality the only way of him finding out for real is to ask you.  I don't know how many times I've had women telling me that other women fancy me, sometimes I only half believe them and others times it is just to awkward a circumstance to ask a woman there and then.  Sometimes you even get the women themselves being present at the time and saying, no I don't fancy him, don't lie.  

Most men are just not that good at reading signs or taking hints or making judgements that women fancy them. Women have a habit of changing their minds often too, so basically the only way he can find out is to make a move on you.  Men find they have to tip-toe around before asking a woman out because if they fly at you quickly and without a thought, it will make them look and feel desperate and they will not be prepared about how to react to the situation.  The way that he will really find out whether you fancy him is to spend more time with you.

And then one day he brought up the subject of masturbation and we ended up discussing it for hours. He said he'd briefly talked about it with his best (male) friend but I've never discussed it with anyone else so I felt really close to him then. Why should he bring such a personal subject up?

I've got to admit I would never risk bringing up this conversation with a women I may fancy.  Most women would be creeped out if a man they didn't fancy started discussing masturbation.  Therefore this is either him trying to put forward a sexual based subject, in order to make you think of him in a sexual way or he is just having a bit of fun and talking to you as a friend and seeing how far he can push the conversation subject.  This is just too much of a risky conversation to be having with someone he fancies, so it puts doubts in my mind that he fancies you.

After I left, I sent him a txt saying that it might not be a great idea to discuss things like that in future as it made me really turned on. He replied a bit dirty and we ended up sending each other several naked pics and msgs to arouse each other while we masturbated. But just before we did he said that it didn't mean anything and he only saw me as a friend.

That kind of makes me suspicious.  As if he sees you as a sexual object on the one hand, but just wants you to be friends on the other hand.  It kind of indicates to me, he wants sex with you, but without the relationship.  He sees you as an alternative to viewing porn on the internet, you can turn him on, and he doesn't have to make any effort to take you out on dates.

Men can be turned on by naked pictures of any woman, whether he finds her attractive her not.  So whether he finds you attractive or not, sending naked pictures and naughty messages would be a big turn on for him. 

For example if you find the e-mail addresses or mobile numbers of 10 men and you sent a naked photo to them, pretending you know them.  I bet you would get a very good response back, the only thing that might stop any of those 10 blokes is if they think it is maybe a scam.  Nevertheless if none of them thought it was a scam, I would bet you get a 100% response rate.  Not all ten of those men, is likely to find you attractive but when they see naked photos of you they will certainly want to know what more they can get.  That is just an example of how a man's mind works when he sees naked photos of a woman.

Why would he be able to talk to me really personally and be a good friend but seemingly be attracted to me without wanting to go out with me?

I really can't see that makes sense at all.  It seems that you have found a guy that is playing a game, with you as his sex toy.

The next time I saw him after the sex texting it was really awkward between us but we didn't talk about it. We agreed (through txting) it wouldn't happen again. But it has happened a few times since and then again each time we say it won't happen again cos it'd affect our friendship.

It's like a drug, you both can't resist it at the time because it is enjoyable and exciting and goes against the usually boring daily routine.  Nevertheless when he is thinking straight (not thinking about sex), he realises that it is the wrong thing to do and he will probably feel guilty about it.  It does indeed look as though he knows you fancy him now, and he seems to be abusing that power over you, to get you to do things you wouldn't normally do.  He is living out one of his fantasies by having you do what he wants you to do.

This story just reminds me of a one night stand.  You get drunk and end up having sex with someone you don't fancy, then wake up feeling guilty and regretting it.  Then you make the same mistake again next time you are drunk.

Plus we're not seeing each other much at the moment cos of circumstances so we're mostly just having text sex. Then after the last time, we said that no matter what it wouldn't happen again.

However, last weekend at 5am I got woken up by him (presumably drunk!) demanding to see my, shall we say 'private parts'.

Quite predicatable, when you are drunk you lose all your inhibitions and don't care about other peoples feelings.  Most men can control their sexual urges when they are sober (you might not believe that but it's true, we have far more sexual urges that we actually display), but alcohol turns off the reasoning part of their brain (and turns off the reasoning part of a woman's brain as well).  And therefore you don't get any reasoning out of a drunk man.

I kept saying 'no' but he was really insistent because he wanted a "good wank" and in the end I gave in - I got nothing in return and no apology the next day for waking me up.

That's it he is just using you and you are get nothing at all from the situation.  You may have been getting something out of it initially when you were mutually masturbating with each other, but now he is pushing it a step further and just selfishly getting off with you without you getting anything out of it at all.  He might not remember it the next day when he sobered up, but more likely he will remember it, but finds it so embarrassing and nasty that he doesn't want to apologise or remind you or remind himself of what happened.  Plus he might want to do it again, so he is not going to apologise for it.

I've not spoken to him since and I'm really pissed off at him. Is he just treating me as his sex slave, thinking cos I fancy him he can do what he wants and demand as he likes?

Yes, unfortunately you are right to be pissed off with him and you have picked out exactly what he was doing.  I have read this message you have sent in chronological order and answered your message in chronological order and as you can see from my comments above, I was coming to the exact same conclusion that you are coming to here (if you pardon the expression!).

Does he just want sex? Or could he possibly want a relationship eventually?

It's possible that he would want a relationship eventually, though I would think this is unlikely.  He has taken so many risks with you, I really don't see that he values you as a long term relationship prospect at all.  But really this guy has treated you so badly, that you really shouldn't entertain any possible chance of a relationship.  It is unlikely to work long term, because even if he changes the way he treats you, there is always going to be the memories there, of how he treated you early on.

And what do I do now? Do I confront him with this and my feelings?

I don't see any point in confronting him with your feelings.  You don't really want to know this guy anymore (at least I would recommend you don't want to know this guy anymore).  By confronting him you are only going to create an atmosphere everytime and confrontation everytime he is in the same room as you.  And he might end up using various excuses and feel he can win you back by talking you around.  This will again lead to you being persuaded to be his sex toy, when he is in raunchy mode.

Or do I just put it behind me and try not to give into him again?

That is exactly the best way to deal with him.  You want to find yourself a decent man, get over him (it doesn't seem like there is much to get over to me, he didn't provide much to you at all).

Sorry this has been so long, but I can't really talk to anyone about this as it's so private and I'm really confused!