And there are very good reasons for this Orthodox approach, other than an attempt to spoil everyone's fun. While I'm sure that many people can supply anecdotal evidence of happy and successful marriages between people of differing beliefs, it can still lead to conflict. I saw it firsthand growing up, and it is extremely important to me that my husband and I share a common faith, even if we approach it or express it in totally different ways. My faith is the deepest and truest thing about me, and I wouldn't want it to become a source of conflict or compromise. Marriage is difficult enough without the added baggage.
Marriage is also, in the Orthodox understanding, a spiritual journey, with the couple helping each other on the road to salvation.
I think St. Paul's idea about being unequally yoked is right on, although since we mostly don't use oxen anymore, it does lose some of its immediacy.