How can you expect to hold up a relationship if you don't believe in the same basic ways of life? Can you imagine a Christian visiting his muslim wife's family to celebrate ramadan? That directly defies Christian religon. Thou shalt not bow down to idols.
I'm sure the Jewish people would have no problems celebrating Easter.
Then ofcourse there's a bit of black sheep syndrome amongst strongly religious families. It is fairly likely that the couple would be shunned by their own family for the choice of marrying outside the faith. (I have a muslim friend who said her parents would basically disown her if she married outside Islam.) That's just how our society works.

Oh, and this was really annoying me. It's beliefs not believes. The latter is a verb not a noun!!!

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Why can't people be more like me? Anti social, sociopathic, and a total jerk even to friends...

but atleast not afraid to speak their minds!