I really don't like the idea of a case by case review - if we're trying to anticipate every unhappy scenario, personal grudges or something might be imagined to come into play.
While I totally support the idea of same-sex couples being monarchs, I also don't think we necessarily have to have "couples" in the sense of any kind of long-term relationship. I like the inspirational idea, but I also kind of like the idea of a completely "marriage of convenience" equivalent, which would be gender-neutral in this case. Someone who is a good fighter would be on the lookout for a consort who is good with business details, can safely drive all night after an event, has a lot of period camping gear in his or her basement, etc. Strictly based on practical considerations. That would be not have been uncommon in the middle ages, after all. ;-)