It's interesting you don't feel bad about it. If that's how you feel fair enough. I suppose the question is, if you were the girlfriend, and somebody else had sex with your boyfriend?
It's also interesting how you argue this might be because he came onto you? Is it fair game then do you think? If a guy with a gf comes on to you, and he's hot, is it fair game to have sex with him? Maybe some girls see it that way.
Don't beat yourself up. You've crushed your friends relationship, but maybe you needed sex and needed to be ruthless to get it? Sometimes you have to look after number one, especially if others around you are getting sex and you aren't.
Some mixed thoughts for you there.