This is actually a really cool variant solution to a real historical problem, wherein either primogeniture or other profoundly shitty customs led to wealthy parents having insufficient resources to provide for all of their children in a manner consistent with their station.
Historically, the Church and its widespread monastic structure functioned as a dumping ground for second/third/etc sons and all the daughters one can't afford to marry off adequately, with the military eventually picking up the slack for the former post-Reformation to the point where it's been argued that the need for something to occupy these dispossessed sons played a role in Europe's ongoing conflicts between its nations and the eventual push of imperialism and colonization over the rest of the world.
In a world where homosexuality were more accepted, it would offer a new option: spare a comparatively-small outlay of resources from the main family fortune to equip a house and accoutrements, which would be reabsorbed into the family as a return inheritance in a few decades, and contract a marriage which would be deliberately unable to produce legitimate offspring.
You get the advantages of creating marital ties with another wealthy family, the people married therein have a spouse and the status achievements that go with marriage, and the risk that your child goes off and marries someone unsuitable or inconvenient is removed entirely, as is the risk that they could marry someone and have legitimate, inheritance-claiming children with them. Sure, they can have affairs and thus get children if they're married to a same-sex spouse, but those children cannot be passed off as legitimate issue of the marriage, and so they pose less of a threat to the the main body of the family's wealth.
And, thus: perfectly reasonable reason why your pseudohistorical fictional characters can find themselves in a same-sex arranged marriage!
