I think that if I did marry someone outside of my religion, it would have to be someone who was okay with a Methodist ceremony. But that's strictly for practical reasons - my father is a Methodist pastor, and I'm pretty sure he has my ceremony planned right down to which of his friends he wants to officiate and who the organist is going to be. So, if I wanted to keep peace in the family, I wouldn't be able to marry a Catholic, for example.
Personally, I don't care about the ceremonial side of things so much, it's more a question of values. I'd marry a Satanist, and my father be damned (figuratively speaking), if he/she had the qualities I was looking for and valued the same things I do.
As for lines I wouldn't cross...I would probably shy away from marrying a fundamentalist Christian, just because we're too different in our worldviews.
And dating outside my church. I've done it. The whole religion aspect wasn't really a problem, although the relationship ended badly for other reasons.
As for the friendship thing, I love making friends who have different religious views than my own - I don't think I have more than one or two Methodist friends at my school, even though it's a Methodist college. It's totally different from marriage, from where I sit.
