I think the problem here, is that we're trying to discuss a multitude of separate issues with separate rationale, as if it's a single issue.
Dividing them up yields these arguments from my side:
- Same-sex relationships (incl. sex): None of anybody's business. This should be respected, it's a private thing that nobody else should be meddling with. This also applies to other 'unusual' or controversial topics in the same category - as long as nobody is being harmed, this is not anybody's business. Harm should be provable. More an ethical point than anything else.
- Same-sex marriage: Marriage being a legal thing rather than a religious thing (certain legal/tax benefits are associated with it, etc.), it should be unconditionally possible. Regardless of sexual orientation, gender, or any other metric that doesn't directly relate to the benefits provided. If marriage were a purely religious thing (but even historically it wasn't), then things would be different, of course. It's the fact that it's backed by an organization with power of enforcement (a government) that makes equality critical here.
- Same-sex adoption: Again, a legal thing. There exists no evidence that this in any way impairs the development of a child (says the American Psychological Association, and other relevant organizations), thus there is no valid reason to prevent this from occurring.
A government has the obligation to care for its citizens equally - it's the obligation that comes with their power to enforce legislation. Thus, where it concerns legal matters, it's not acceptable for a government to treat same-sex couples differently. A church is - in a properly church-and-state-separated nation - a private religious organization, and thus carries different obligations. This is pretty much the entire point of that separation.
Again marriage isn't a religious thing, unless you are religion adherent. It's something religious institutions stole from individuals under terror terms of hell and control.
Again marriage isn't a legal thing, unless you are a worshiper of the State. This nonsense of government "marrying" people is legal non sense. It's akin to registering a dog. It's an inventory.
Marriage is a personal contract between two people and historically between a male and a female. But it could be between a person and a sandwich, who am I to judge who a person loves?
This also applies to other 'unusual' or controversial topics in the same category - as long as nobody is being harmed, this is not anybody's business. Harm should be provable. More an ethical point than anything else.
It's extended on unusual side to absolutely every abstract deviant idea you could create and more you never imagined. Mark my words this decision will march the fringe folks into demanding 'rights' and being recognized as 'normal' just cause otherwise such is discrimination. Much of law is based upon discrimination and victimizing people who at liberty should be left alone by the State.
The word you're looking for is consent. Animals and children cannot give consent. I don't quite understand why this concept is so hard for the religious and right-wingers to comprehend.
Sure they can and do, which further complicates the issue. How many times have I heard teenagers say about how unfair something is and how they are discriminated against them? Daily thing. You know what? I agree with them. Plenty of ageism. Arbitrary numbers equal graduating to be allowed to give consent is more government nonsense in the lives of people stuck with their jurisdiction and forced to be subjects of the State. Minors can enter contracts and are able to do at various ages. It's not very common, but exists.
Go back in time and people were teens and on their way having families and all sorts of stuff. So this oddness of like 15-18 we put children through is insanity and ageism. Parents and State ganging up on kids. Animals if they aren't interested, surely are capable of defending themselves and are capable of inflicting grave harm on a silly human. Surely, hospitals probably see many cases of deviant behavior cured through by the force of an angry animal.
Consent is never given from non speaking entities and objects under such a tight definition / view. As such I suspect they are unable to testify and claim victimhood except in the insane cases where the blind can see the brutality of what occurred. Speaking here of sheer violence.
See I don't agree with the fringe behavior, but I see the logic and slippery slope of government butting in on such matters. Figuratively prying Pandora's box wide open.
At law, to have a crime it always has been the need to have a victim, for the accused to have right to face the accuser. This recent decades of proxy by the government standing in and representing the interests in lieu of victim has led to mass horrors. The drug war is a prime example of victim lacking often, yet people remain harshly punished for what may amount to self harm and well within their human rights to self medicate. Slippery slope again where one allows government an inch and they fit a mile in.
There is no real need for marriage and both church and state should be out of the business of marriage but both profit handsomely from their rituals. I don't even think marriage by local government is even called marriage. This is a recent redressing of the ugly bride. Forever I recall it being called Civil Matrimony. This likely has a distinct legal definition and implications that are not 1-for-1 overlay of marriage.
There are also among the States of the United States and elsewhere various forms of Common Law Marriage that exist. The most shocking in places is the government deems a couple (male and female) to be Common Law Married after a period of cohabitation. In some places this time is woefully low. Two people of opposite sex playing roommate could be wrongly roped into such labeling in as little as a portion of a year.
PS: I don't support special status and tax gimmicks for married folks either cause that's discrimination against unmarried people.