Not that it doesn't belong per se, but it's just that it's not necessary and hard to implement. Bioware did think about adding polyamory relationship in the game like you joining an existing couple, but it would be very hard to implement. Plus, it doesn't really give a representation to any group, but more like representation for a lifestyle choice. It's not a necessity, but if the did include it, it would just be because the characters are happened to do it like Iron Bull with BDSM. Something I wish they didn't include since it make the relationship to be very one-sided and presumptuous, Iron Bull knows what you want, Iron Bull takes control. It's basically take away the imagination and seem like a relationship based on domination. At least for me. It may or may not have impact the romance where it's basically pretty jokey.
Personally I DON'T think these sort of kinks belong in the games, mostly because they are already splintering our choices by making love interests who only accept male PCs or female PCs, and in some cases, only certain races. If there were more opportunities for genuine romance (instead of having to figure out or force yourself to play a certain type of character you don't identify with or enjoy playing just so you can romance a particular character) then okay, maybe. Like if there were six love interests and all were open to any PCs, then fine, make one into BDSM and one into polyamory (I guess technically you have to make two of them into polyamory).
But when choice is very limited, and romance isn't supposed to be that big of an aspect in the games anyway, I think giving us all similar basic romance choices (maybe each PC gets a "cool and badass" romance and a "gentle and sweet" romance) is the best way to go.
No kinks, alternative lifestyles, etc. Just romance, same as everyone else.