Let me speak from my perspective to maybe justify what he's saying. I'm in my mid 20s now. I used to sleep around a lot when I was younger and probably had slept with a similar amount of girls by the time I was his age. That was just me being a young and single, good looking lad who knew how to talk to women - the circumstances were right and I loved being adventurous.
Now, I'm a bit older and I've settled down. I've never cheated on a girl, in fact I stayed single because I knew I wanted to sleep around, and to try and have both is unfair on your partner - nowadays however, if the right girl came along I'd happily settle down with her with absolutely no intention of sleeping with anyone else, my days of promiscuity are behind me.