NSFW: Would you be the lover of a man who is already in a relationship?

If you were propositioned by a guy that you have ‘seen’ in the past for a for a secret rendezvous. Both young, no responsibilties.
If you had the offer on the cards, for no strings, secret “coital’s” – would you?

So i have been asked to edit this question: If you thought that he may have a girlfriend, but it was nothing serious – would your opinion change?? Is it the other womans responsibilty to not allow for the man to cheat in his other relationship?

Please refrain from any judgement like: he is an arsehole and other negative and unhelpful yet obvious remarks. This question is NOT about the girlfriend, but rather the ‘Other Woman’.