The only excuse I could think of is extended time out in space of a decade or so.
Right. The original intention of TNG's creators, which unfortunately got lost due to the revolving door of producers, was that the Enterprise
-D was a deep-space research vessel designed to go as much as 15 years without returning to a home port. It wasn't supposed to be doing diplomatic milk runs to Federation members and neighbors like it ended up doing for most of the series, but instead was supposed to be far beyond the fringes of known space, years' travel from home -- somewhat similar to Voyager
's situation, really, except on purpose.
So think about it. How many volunteers would you get for such a mission if everyone had to leave their families behind, or defer starting families, for 15 years or more? That's a huge chunk of a person's entire life. It's not something very many people would be willing to give up. And for those who did, going a decade and a half without family life could be extremely stressful and harmful to crew morale and cohesion. The only way such an extended deep-space mission could really be feasible is if the ship isn't just a ship, but a whole community, a small, self-sustaining city in space.
Also, keep in mind another thing that later producers forgot: this was meant to be a research vessel, not a military one, so its crew included a large complement of civilian scientists. It wasn't just Starfleet personnel and their families, not as originally intended. I like to think of it as a university village in space. It was supposed to be primarily a research vessel -- with enough Starfleet presence and weaponry to defend it if it became necessary, but never intended to go into combat except as an absolute last resort. Maybe you could find enough military personnel willing to commit to giving up 15 years of their lives, but you'd be harder-pressed to get civilian scientists to join such a mission.
And then there's the other abandoned element, the ability to separate the saucer and leave it behind with the civilians aboard while the Starfleet personnel went into battle in the engineering hull.
So the problem wasn't with the idea of families on the ship. That idea was very well thought out in terms of the creators' original intentions. The problem was with the way the later producers screwed things up by ignoring those original intentions and turning the E-D from a ship exploring strange new worlds to a ship that spent most of its time hanging around known space and going on diplomatic or political missions -- and bringing the saucer along into combat because the only miniature they had that could separate was too cumbersome to use regularly. And forgetting the civilian presence altogether except for Keiko.
It's a problem with the intentions of the original team being at odds with those of later writers/producers, though people like Ron Moore and Michael Piller seemed aware of the issue but unable to figure out how to resolve it. So the families would appear occasionally when relevant to a story, but would vanish without trace if not.
IMO they'd have been better off making it a plot point after Best Of Both Worlds
, ie. that it was becoming increasingly too dangerous for this kind of civilian presence to be aboard ships. Instead they kind of acknowledged it right up to the death of the Ent-D, and then quietly tried to ignore it (IIRC Janeway acted like the very idea of children on Starfleet ships was unheard of when Naomi Wildman was born; and the Ent-E famously ditched any hint of there being families on board as well).
Re: the Enterprise originally being a 'deep space' exploration craft: ISTR a season one episode (Conspiracy
?) sees Starfleet command being very surprised at the arrivial of the Enterprise in Earth's solar system, with the admiralty even saying that the presence of a Galaxy Class ship in proximity of Earth was rare. Later seasons it seemed like they were always
going back to Earth!