What's different about startups is that the demands are unpredictable. A couple can steel itself for law school. And lots of marriages do break up over medical careers.
Academia is worse than startup-land for relationships because it's almost certain that you are going to have to move around a lot, and quite possibly to a college town in the middle of nowhere. Startups you don't really have to move, and even if you do, most of the places you would be moving to are pretty nice.
What about sports though? For example football players transferring teams, suddenly being apart from their families, having to train in quick bursts, suddenly being in the playoffs/world cup/etc.
Seems very unpredictable to me. Plenty of other professions that are like that, I'd expect Musician, Actor, Artist to be pretty unpredictable/high stress.
Pro athletes make enough money to alleviate most of the issues that their lifestyle brings about. Plus, they are pro athletes. Yet they still get divorced. And sometimes they don't. Musicians, actors and artists get divorced all the time. Or not.
Artists on the other hand never make any money, they are busy all the time, and have a very predictable lifestyle (always broke, always working). They usually get dumped because their SO wants to live in an apartment that has its own bathroom and shower.
In every interesting profession, your professional life eats into your personal life.
Imagine being an astronaut and telling your wife you need to be away for a few months and that you have to train extra hours.
If your career is interesting enough and eats up your personal time, it probably is ok with your spouse.
However, be a truck driver and try explaining your spouse why you need to be away 4 days of the week.
