Why would he say this?

He probably told you what he REALLY thinks about getting married and having kids, and was only putting on a 'tough guy' front for one of his buddies. The fact that you said he 'acts like he doesn't need anyone' leads me to believe he's a bit of a cocky s. O. B. , so he probably was trying to make it seem like he was never going to be tied down by a female in front of his friends to retain his reputation, but when he was alone with you he probably let the truth come out. Typical guy, but this does show a bit of insecurity.

Because he actually Does want kids and to get married. Maybe the seriousness of the conversation got through his defenses and he spilled his guts to you that he actually does want kids. I don't know any guys that would joke about this, or so easily, especially if he, as you said, suddenly got serious.

Maybe what he says to his friends and what he really thinks are two different things. Some guys feel the need to act like "players" around their friends (particularly male friends) and often it's just not considered cool for a young guy to want a family.

If he thinks you're already going out with someone, he won't be scared to tell you that he wants kids because you're not likely to call him on it or tell anyone like a girlfriend or potential girlfriend might.

These are just ideas that spring to mind, but in reality his motivations may be completely different. The only way to really be sure is to try and subtly ask him.