I'm also lucky enough to have a phenomenal, supportive girlfriend. I live with her, so I get to see her fairly often, but at the same time, we usually both leave the house in the morning at the same time, I'll come home at 8, and then head to the coffee shop till 12, so we don't get to spend all that much time together. Luckily, she's still in school, so that helps. I generally make up for it by ensuring to spend some time with her on the weekends.
We had a year and a half-ish in before the startup stuff started, so we'd already had a fairly solid foundation in place to build off of. Also, (even though I try not to talk about this online, but it's relevant, I'll make an exception because I like you, HN) the fact that we have an open relationship helps take a little load off of the responsibilities as well.
The other big factor with women and startups is finding a girl that's okay with splitting the check, going bike riding rather than going to the movies, playing board games rather than going out to dinner. When you're living on subsistence wages, it's quite hard to do the whole "the man pays for everything" bit. It should be impossible. I hate to say that it's hard to find a girl like this, but it kind of is.
My life would certainly be significantly worse without her, though. There are enough unstable factors with startup life, and having that stable support really, really helps.