Every guy is different. Some will be goofy, some are never goofy, some change, some don't. As for becoming more mature and responsible after having children, some do, some don't. While you can't know for sure until it happens, I think the best way to see how they'll act with children in the picture is to see them around children. Ideally, if you could get a child to watch for several days, so you can see what happens when it stops being "fun and play," you'll probably get some idea.