Stan is a terrible example. Stan wasn't much before he had a wife and kid, either. And he didn't ditch his wife and baby to "focus on tennis." It was to focus on Donna Vecic, we later learned. Agassi/Shields is an even worse example. Two-celebrity relationship, oh, and he had a meth addiction which broke up their marriage. (This from her book.) I bet that didn't help his tennis.
I believe I have said that priorities change with kids, of course they do. I don't think simply getting married 14 years into a relationship changes anything, though. I don't think everyone can pull off schlepping 4 kids around the globe as well as the Federers have, but I don't think one or two small ones has to affect a rich athlete's life a lot. A not-so-rich one more. Priorities, yes, though, and so perhaps motivation for tennis, as well. Or not.