You haven’t noticed how opening the floodgates on gender many decades ago is already leading us in this direction? There is no stopping point - which might be okayish once we hit the Transhuman era and most of the negative consequences are removed, but before then there will be consequences and not all of us support atomistic individualism.
At issue is that rightists cannot tell what isn’t a risk and leftists cannot tell what is a risk.
Homosexuality is not contagious and isn’t particularly harmful. It also doesn’t make children. It doesn’t even really alter the dynamics of the straight dating market much. However, it would be a problem if it hit 20-40% of the population because as a species we depend on reproduction. (So if the situation were different then I would have a different opinion. But it isn’t, so gay marriage promotes membership in society, family, etc.)
But I will hold the line on polygamy, which is really the harmful polygyny when normalized, and we can see what happens in countries where it is common. I will hold the line on cousin marriage, which not only increases genetic risks but undermines nations. And of course, the taboo must be held on pedos.
These things have consequences far beyond the individual, and if your response on cousin marriage is “oh we’re going to make it legal but don’t worry, no one will use it anyway”, then why make it legal in the first place?
The ideological tools used to break down the barrier will prevent us from doing anything about it, like banning cousin marriages from producing children. On some level you must know that.