Marraige is a human institution.
Yes, between a man and a woman, only.
Why?
Because God created marriage between a man and a woman for the purposes of procreation.
So then should infertile [heterosexual] couples not be allowed to marry?
If you're using that argument, you are saying that infertility and homosexuality are on in the same category. So you're calling homosexuality a "disease" (that's not the right word for infertility, but I can't think of anything else, but you still get the point)
I know what you mean, that infertility (unlike homosexuality) is an abnormal medical condition and therefore the two shouldn't be put in the same category. But then again it is not abnormal for a 65-year-old heterosexual couple to not be able to conceive a child, yet they would not be barred from marrying.