Wow, sounds like you have a lot on your mind !!
I would DEFINITELY not recommend getting married JUST to have a child!! Let me tell you from personal experience (I have an 11 month old baby), he is the love of my life, a wonderful baby, but ALOT of work!! If you have a child with someone you don’t love or even don’t like for that matter, you are bonded to that person FOREVER regardless if the marriage works or not. If someone goes into a marriage knowing they're not going to be faithful, knows they're going to have a lot of problems, etc. then why would you want to bring a child into all that? IMHO I truly believe that having a child is a privilege not a right.
But with that all said, I think people (including guys) can definitely change their ways, be faithful, and have a successful marriage and relationship as long as they WANT to. I have known quite a few people that were, well, let’s just call them “party animals”, then they calmed down, got married, had kids, and have been happy. But they wanted to change their ways. If they went into the marriage not wanting to be faithful, then I think there would be a lot of problems.