These are actually completely separate and unrelated questions.

I've seen lots of men who claimed to love the women they cheated on.

As for why some men cheat, in my observation, it's the same reasons why some women do.

Another thing that they have in common, is that most cheaters wont tell the truth when you ask them why. They will come up with some silly excuse, such as that "it's part of being a man," or "because I deserve more than I was getting from him/her."

It should be obvious, but it needs to be repeated:

If someone lies about being faithful, they will lie about why they cheat, too. especially to themselves.