Very old, traditional same sex marriages? I don't think I've been reading the same history books as you have. Especially for marriages used to forge alliances, a key part of the process is the production of an heir, to solidify the connection between the companies (if it is a family business), regions or countries. Historically I think the prospects for a straight political marriage dropped off significantly if someone was believed to be infertile. How many people would arrange a marriage for a child to strengthen a company or country, knowing their legacy or family tradition / family rule would come to an end? Even with extended family members, or third or fourth children, where an heir might not be relevant, the statistical probability of one group having an openly gay member wanting to form an alliance with another group also with an openly gay member seems pretty small. It isn't like straight arranged marriages start off with hundreds of prospects.
It isn't known if the main character's gender will be an option yet (currently it is male and there are princesses to marry). If Larian does add that option and a bunch of princes, that will double up on that section of the game, and make testing more complex, etc, using resources that IMNSHO could be better used for other features. Allowing a choice of sexuality for the main character would greatly increase the amount of work required, unless they just assumed everyone in Rivellon was bisexual.
