Why did John have a sexual affair with Abigail when he knew that she was too young and there were a lot of girls his age?
Ah, that's one of the core questions of John Proctor's character in this play. Unfortunately, like several of the key elements of this play, we don't get to directly observe the action, and we can't see into his mind. All we have to go on is what he says, and what Abigail says, and perhaps the general atmosphere and general human psychology. Abigail talks about being a "wild thing." Perhaps he was drawn to her wildness. She also accuses his wife of being cold; perhaps John was frustrated in bed. Perhaps it was the boredom of a small community. Perhaps it was the fact that he should have power over her, and there is a long history of men who have power using it for sex. Perhaps it is that she wanted him so badly that he was tempted.
I think part of it was opportunity. Abigail had been a servant girl in his house before Mary Warren. Elizabeth Proctor (John's wife) was sick at some point and it's also mentioned that Elizabeth was cold toward John. So he's lonely and Abigail is there in his home. The wild nature of Abigail is shown throughout the play and I think Abigail probaby made a pretty good effort at seducing John.
It is important to remember that these are Puritans.Most women conformed to the proper image-never alone with a man who is not their husband, spending time caring for home and family, and fearing the Lord.
It is quite likely that Abigail was the only woman in the community that dared to be aggressive in going after a man's attention.
John was probably wanting something new and different, and Abigail offered that. He and his wife were probably not getting along at the time so he decided to get his male needs else where. It's not that he didn't love his wife anymore, it was more of a physical need that needed to be filled. Abigail was young innocent and beautiful which made him all the more attracted to her.