The unfortunate truth is that all men should be able to take rejection and move on with their lives, but few seem to be able to. I don't really think that it's your problem that his feelings are hurt just because you don't want to fuck. I could write you an essay of excuses on why you shouldn't have sex with your roommate beyond your own personal reason, but the fact is this: Having sex is a choice and casual sex shouldn't be taken so seriously.
The two of you are adults and should be able to live your lives together without the stipulation of sex keeping things calm between you two. I have a lot of harsh words for a man who can't take rejection and even worse words for a man who uses pity from it to get what he wants. Maybe he is shy or embarrassed, but rejection happens 9 times out of 10 in life so he needs to learn to deal with it. I don't think that his problem is your problem, but if it's really affecting you negatively, have a serious talk with him. Tell him it wasn't personal and that you're just not attracted to him in that way. It doesn't mean that you can't be friends, but if he's going to take one rejection so personally, it's going to make it hard to be around him.
