I mean I wonder how you can be with someone and claim to love that person while at the same time believing that this person is destined for hell. And I've even encountered women that looked at me with genuine fear in their eyes when I told them I didn't believe in God. For them it's code for amorality.
This is a good point. I have the opposite reasoning, if I were with a theist (which I am not, my girlfriend is agnostic/atheist), they would certainly not be able to put up with my frequent moaning about how annoyed the school chaplain is making me. (for those who don't know, my school has compulsory chapel, which I usually don't mind going to...)
He tried to say that Jesus loves gay people this morning, it was bloody ridiculous. There are well over ten counts specifically in the Bible condemning the act of homosexuality and saying that homosexuals are destined for hell...
Utterly ridiculous, fallacious and infuriating to listen to. I did NOT approve of his argument.
[see above 3 lines for an example of why a theist would leave me pretty damn quickly

]
I know there are exceptions, but I know theists can find it hard to live with people who believe differently to their religion, especially if they get argumentative, and I know for a fact that I am pretty riled when someone contradicts my view with logical fallacies that are unsupportable...
My mum is a Jew, and my dad is a staunch atheist, they get along fine, religion just doesn't get discussed, although my dad did not really approve of mum taking me to Sunday classes at synagogue. It was hit and miss whether he even attended my bar mitzvah... He did though, and I was pleased he was there, even though I now am not entirely convinced of what I learnt.
Basically, I'm trying to say that I think there are a lot more obstacles for couples of differing faith to overcome than same faith couples. Not saying that people can't do it, I know that many of the people on this forum have got partners and close family members with different views.