How should you feel about being the girl a guy cheats on his girlfriend with?

I feel very bad hooking up with a guy, and later finding out that he has a girlfriend. I don't expect to have had asked while getting acquainted at the club, whether he has a girlfriend, because I've always felt that if someone comes on to you, their behavior is meant to imply that they are single.

So, having experienced my second time of being the person whom a guy cheats with on his girlfriend, I just don't know how to digest it, process it, receive the realization that it has indeed happened to me and I have been either the culprit or fair accomplished depending on how you like to place blame.

How should I take these things? I feel lousy, and wish I could feel positive about it some way and somehow. How should I feel about these kind of things? How would you cast your personal opinion about it?

Updates:

What kind a women do guys generally cheat with? :( Apart from attractive to some degree,... Does this mean I exuded some quality that made me look like a good target? Like...sluttiness? Oh GOD. Does this mean he felt I was non-respectable? Ugh!!

For the record when he told me I thought "ughhhhh! What the fuckkkkkk! Not againnnnn!!! Why me why me, this is the first one night stand I've ever actually had and behold my lousy luck!" And I immediately felt shitty because even if I was breaking up with my boyfriend, I wouldn't like to be cheated on, and I'm a girl's girl, and so I'd hate to do something unfair to another woman.