The fact of the matter is, even if he was in the wrong (and I don't think he is), she was aiming to fuck up his life.
I'm going to assume that she wasn't so fucked up to knowingly pin another guy's child on him. I'm going to assume that, with all of her whoring about, that she didn't know who the father was and simply chose him as the one that she thought was best. If she's that big a whore-bag to him at the beginning of their marriage, how do you think she'll be treating him years from now? At best, he's be tied to her via alimony and child support. At worst, she stay with him and be a complete douche bag.
Simply put, he wanted to hurt her feeling, she wanted to fuck up his life.