Quoting taylorgurl:" Or he tells all the girls this just to get their guard down. I would move on because I doubt he won't cheat."
My ex told me the same thing. That he had cheated on every other girl he was with because he didn't care about them, but of course I was different so he wanted to be open and honest with me. Gave me his email and FB passwords and everything. I never asked him to, he gave them up willingly because he had "nothing to hide". A couple months later, I got a bad feeling so I logged onto his FB and found out he had been dating another girl for the past 2 weeks. And this guy was 31 so it didn't have anything to do with him being "immature".