If a person wants to stop an entire class of people from marrying though, I think that at least on some level, they must be averse to that group. Otherwise why even care.
Religion is a very powerful brainwashing tool. You can't reason with it if they believe that marriage is a religious thing between a man and a woman. This does not mean that they hate gay people but rather truly believe by God's word it cannot happen.
Some countries have child marriage for example, which technically is a class of people as it is embedded in their culture. Would you argue to allow that in a modern civilized world as you are excluding? Sadly it is a fairly complex issue and the morality of it depends on how a country sees it as what is bad and what is good and religion has a part to play in that too.
In reality marriage is an outdated concept that shouldn't be tied to Law/Politics and what should really matter is the legal rights that people have when they choose to live together as a couple. When I got married I didn't feel like I suddenly had all these extra perks, neither dd my wife (perhaps we been together so long it didn't feel different). In reality it is more a celebration of love than anything else these days as many women are now fully self sufficient and employed.
In the past marriage felt more like a woman signing a contract to a man as the woman would be under a man's protection as he was the one that worked and the woman would stay home raising the man's offspring. Go back further in time and it was a way to join powerful families together lol.
PS - It is almost 3am so apologises if I went on a side rant here