uuurggh... I would have a real problem with it. As long as he keeps that shit to himself completely, not mentioning it ever ever in my vicinity or in my child's (childrens'?) vicinity, I could accept it MAYBE. I don't know why, but this feeling of disgust overwhelms me every time I'm confronted with religion whichever it may be.
I have thought about this a lot, too. Pretty much the only problem in my current marriage is this lack of ability to properly communicate and agree. I get that one of the great things about being in a relationship is seeing things from new perspectives other than your own, but there's a limit, right? When you can't even see eye to eye on the basics of life, it's going to create a divide. Even if it's small, I don't want to be 80 and facing imminent death and nothingness from my perspective, but my significant other gearing up for his second wind and his new life as an angel/ghost/"inexplicable mystical energy."
However, in practice, I suspect I'm more lenient. One of the closest individuals in the world to me is wishy washy about the big picture questions, reluctant to dismiss Christianity or deism, but also rarely thinks about it. Still, there is no one on Earth who understands me as well as he does, and, if I were considering him as a romantic partner, I think I'd let slight ideological differences slide in favor of that. But his would be an exceptional case.