If we're going to get all Roman about it, I was really just saying "nulla poena sine lege". There is literally no law (at least in most civilized countries) that forbids two men or two women from performing a marriage ceremony and adhering to all of the customary duties of marriage. Consider the rather bizarre tradition of marrying dead people. This is not in any sense legally binding (because a basic principle of law is that it applies only to living people), but people do it anyway.
IMO marriage (the relationship itself) can be nothing but spiritual. Where the State is involved, it can do nothing but harm. If I were being really cynical, I might suggest the only purpose of marriage is to provide employment for divorce lawyers. But I do realise that's only my opinion, and that some people do crave the approval of Society At Large, or of their elders and betters.
The problem here is that it should not. What I mean is, one should not have to get married in order to get various "privileges" related to social security, taxation, travel, etc. The whole concept is stupid. Another way of looking at it is that these issues are discriminatory against single people in general (or people who the law deems to be single), not homosexuals specifically.
All kinds of people are denied that chance, for completely arbitrary reasons. Single men have zero chance of adopting, even though it is legally permitted in many places. Single women only a slightly higher chance. Very young or very old people will be denied. Being married is not the one and only qualification: the authorities are pretty much at liberty to say "no" to anyone they choose. This is done to safeguard the wellbeing of the child. They probably eliminate far too many potentially-good parents, but they err on the side of extreme conservatism because a mistake could be disastrous.