People simply have more time and energy to devote to practice before they start a family.
yeah, i find that my Aikido schedule is certainly not as rigid as it used to be. my wife and i are expecting a child, i'm getting more responsibilities at work, etc etc.
i don't feel starting a family will affect my training, but i do feel it will affect when i train. i even considered going on my lunch breaks at work, but that doesn't make any sense in the engineering field.
i've seen many married couples train together but unfortunately i don't have that luxury.