I think the issue of kids is a big one here. I can see someone converting in name only, or converting for the cultural aspects, etc., to be with someone, I guess. But then there's the issue of kids. Will you raise the kids to learn both "sides" of things, and then let them choose? Will the one person lie when they ask her what he/she believes in? Does the converting person really think it's OK for his/her kids to believe something that he/she doesn't believe?
Personally, I'm an atheist. I don't care what religion anyone else is as long as they don't force it on me. And I believe that raising kids in a particular religion is forcing that religion on them--so I could never be a part of that. As far as relationships go, it seems to work out best if the person I'm dating (no matter how they were raised) does not see religion as a signficant part of life, and has a faith in science, nature, and logic either in addition to or instead of a believe in God.
"I don't know! I don't know why I did it, I don't know why I enjoyed it, and I don't know why I'll do it again!" -BART SIMPSON