Marriage is for producing children. Even societies like Greece and Rome, with all their homosexuality, used marriage for producing children and combining property.
Although Greece and Rome are often pointed to as some sort of bastion of homosexual tolerance, in many ways they were harsher on open displays of homosexuality than any civilization today. To some degree limited homosexuality was tolerated among the ruling class, mainly because they held all the cards and could pretty much get away with anything they wanted, as long as they kept it out of public view. But it was never considered acceptable among the other classes. Also some of the modern confusion comes because Greece and Rome had very different definition of what they would consider "homosexual behavior", for instance, (not to be to graphic) but two male friends giving each other...manual stimulation... was just considered normal "guy" behavior, not much different than two male friends watching the football game together today. However any sort of behavior that included penetration was considered a huge no-no and looked down upon.