I think you can have happy marriages with people of different religions, but only if both are willing to work for it and make some compromises. My aunt is Catholic. Her husband is a Muslim. Her 2 boys go to a Catholic school, but they also go to religious services at the Mosque. I think it's great that they are exposed to both religions.
My aunt does not participate in the Islamic religious services, no celebrate Islamic holy days. She does however encourgae her husband to maintain his faith if that's what he wants.
So, 2 interfaith marriages/relationships can work, but only if you want them too.