CeLL wrote:
morals are determined buy the majority of a society, so yes in america, its immoral.
Fair enough, but if you go deeper than that, is it truly immoral? From an evolutionary standpoint, no.
I dare people to type "Marriage is a religious" in google, then click the first yahoo answer link. Check out the "best answer", I nearly knocked myself out with a facepalm.
BUT, to answer the OP, I think most of us agree it stems from religious/societal/cultural beliefs.