A few thoughts (and holy wall of text incoming, Batman!):
I have spent the entire day trying (and failing) to fix a small glitch in my personal mod for enabling same sex concubinage that makes it 'literally unplayable'. Stubbornly trying to fix it in a way that won't cause me a maintenance nightmare when the next patch comes out or I want to use some mod that touches the character window.
You see, just like CK2, the character windows displays 'Man' or 'Woman' next to both a concubine's partner, and the marriage type icon, depending on whether they are male or female.
Depending on whether *they the concubine* are male or female, not their partner. The assumption is made somewhere that their partner is always opposite sex, so it gets it completely wrong if they are actually the same sex. That dude he is a consort of? They are his 'Woman' apparently.
Just a localisation change right? Okay a localisation change with some custom logic, which -- after much hair tearing and forum searching -- is doable with the custom_localization stuff. Only not really. This a particular chunk of text doesn't come directly come the localisation system, instead it apparently comes from a C++ function on the CharacterWindow that refers to the localisation internally. It seems it refers to the localisation at a low enough level that the character is not pushable into the custom_localisation in all circumstances where the source localisation is used, resulting empty text being displayed, a logged null pointer exception and 1500+ lines more log of repeated localisation error message.
So, on the one hand I have a certain amount of sympathy with the view that although it is simple to take out a hard-coded check, the known unknowns of cleaning up random subtle glitches that might not even be noticed at first glance, is the sort of thing that gives experienced devs the willies.
On the other hand: Come on Paradox, you put in a whole sexuality system, built a modular religion system that takes that into account with a doctrine slot, *and* a game rule slider that lets everyone change the sexuality distribution. What planet were you living on that you expected people not to get antsy that same sex marriage isn't in the base game, for settings of the game rules where it would be seriously implausible for it not to be a thing? That when we discover it isn't in there we won't immediately try to mod it in, and find you locked it out completely even for mods, getting everyone even more antsy?
I mean, the reason I bothered to make a personal mod for concubinage is that it just seemed implausible to me, even on default, that if you did have a religion that accepted homosexuality and that religion also had concubinage, that that religion wouldn't prefer your gay lovers to go in the official 'possibly not actually for breeding, maybe just for fun' slots. Suspension of disbelief depends on historicity to some extent, yes, but is also depends on being consistent with the world you have actually built not just the one you're trying to simulate.
On the gripping hand, those of us who wrote mods that did super sneaky things in CK2 are maybe one of the reasons Paradox have tightened up their coding practices and have put 'sanity' checks in more places.
I made one such sneaky mod using a buggy unintended side-effect of temporary titles, called 'Founder of the Republic'. It made it possible switch your character to being a Merchant Republic back in the days when that was actually 'impossible', because there was no script command to change government types at all, and the player becoming a Republican was hard coded to be instant game over.
First thing a youtuber tried to do with it was make a Republic with a female character, and that suddenly revealed a whole shuck of utter UI brokeness, caused by him doing this not-supported thing using my super-not-supported how-does-that-even-work method. I'm guessing that even a vaguely popular mod that relies on some sort of glitch is a support and patching problem. If a *really* popular mod (unlike mine way back when) ends up doing unsupported things, of course that mod is going to hit the streamers and the youtubers and the twitterers, and now you're in the realm of bug-for-bug compatibility and tiptoeing around fixing things in case you break the stuff that shouldn't work but does and is heavily being used. And yes I also remember that long series of patches playing whack-a-whole with male pregnancy. So CK3, the new project, resolves to make sure that impossible things really are impossible this time.
It's just such a bad choice of a thing to be impossible given everything else in the game.
TL;DR Paradox, this day job programmer, and long burnt out CK2 modder, can see reasons why you wanted to lock same sex marriage out, but, I'm sorry, not having it as at least a mod option doesn't make any sense in terms of the game you actually wrote. The horse is well out of the barn, and you know you're going to have to unlock the scripting commands sooner or later, right?