Reading all these vicious comments from people wishing harm upon the OP, calling her a bitch, a slut, a slapper, a homewrecker etc and telling her she is a horrible human being... it's actually quite upsetting. You all seriously need to lay off. Forums are for discussing points of view, not bullying.
I personally think from the way that the OP writes and puts her point across that she is an intelligent girl with well-thought out views and an open mind. I know I'm going to get negged to hell for my view on this but this is my view which I have thought about logically, rather than just going, "omg what a bitch you are helping him cheat your such a bad person and I hope you die". Yes, obviously SOME sort of responsibilty is on her to say "no, you're married" and refuse, but at the end of the day this man was quite willing to cheat on his wife, whether the OP actually agreed to it or not is fairly unimportant because the man has already betrayed his wife by trying to bed other girls. He is clearly a cheater and has probably been seeing other girls behind his wife's back for a while. He is obviously an arse. However, the OP has no commitments. She is free - she is not cheating on anybody, or betraying anyone. She has not made any commitment to this man's wife, HE has. He is in the wrong.
I was cheated on once. I was heartbroken and angry at my bf for lying and cheating on me. I was NOT angry at the other girl. She doesn't know me, she didn't have any loyalty to me, she just met this guy and fell for him, just as I did. She seems a nice girl and honestly I just feel sorry for her because I know what he's like.
Cheating is not black and white. However it is morally wrong to betray someone and lie. This is what the OP's man is doing. He is an immoral prick for sleeping around behind his wife's back, but it is not the fault of the person who he is doing it with. He made the commitment, and he broke it. To all the people calling her a "homewrecker" - the OP is not wrecking their home - this guy is, by cheating on his wife, lying and betraying her.