You! Are the man! Or woman, or whatever. Do not let your beliefs. (Even if we are talking about empirically justified, rational beliefs.) Fuck up your relationship. Now, it may very well be that there are underlying issues in your currently significant others grasp of reality that might one day infringe upon your shared happiness. Really. But dear God, don't screw it up because you can't believe in fairies, or she thinks atomic powered spacecraft are a terrible idea, or because you believe in strong AI and she doesn't. Or because you're sure the singularity is nigh, and she hasn't a clue what you're on about. Don't lie to her, don't pretend, don't play games, but if in the short run you have to believe in cavemen astride velociraptors, fuck wait, I think I have decided to believe in that now, just how cool is that anyway?