The noble who already has heir and spare wishes to remarry but there mustn't be any additional heirs: non-reproductive pairings only for second spouse
We, a friendly set of neighboring families, cannot afford to split the estates between all our children and have them survive: enforced polycule situation between 1 kid per neighboring family, effectively rotating estate control without breaking any up.
We, a single family, cannot afford to split the estate between all our children and have it survive: ONE kid gets to be reproductive, and everyone else's inheritances will be absorbed back into the main estate when they die because no legit heirs.
The monarch has decided you, their presumed heir, are no longer worthy to carry on the bloodline and you have been sentenced to Gay.
You, our heir, are trans and we need a next gen heir, so you have been sentenced to Gay.
For Reasons, we have decided it would be embarrassing for it to be known our non-heir child is barren, so, yep, sentenced to Gay.
Rich family wants connections, noble family wants money but doesn't want the bloodline being "contaminated" by offspring: Gay marriage alliance only.
Two kingdoms make a peace treaty but one really isn't feeling like keeping the peace long term: sketchy kingdom sends a groom instead of a bride or vice versa, and everyone in the receiving kingdom quietly shits themselves at the implication that peace shall not bear fruit.
(Two kingdoms make a peace treaty but both aren't really feeling like the peace will last: they both do a "swap" by sending a trans spouse of the specified gender and accidentally undermine themselves via reproductive couple. To save face, each claims to have been successfully spying on the other, thus forcing the war they wanted and creating a star-crossed trans couple)
Two kingdoms make an HONEST EFFORT at a marriage based peace treaty but there was some miscommunication about their enby representatives (one renamed themselves after a sibling who died when they were kids; one has cis siblings of all one gender and got lumped in), PLEASE don't declare war again
Some demographic weirdness happened: your options are gay or single.
It didn't start gay, but transitions happen.
Listen, the cursed bloodline has to end with you, but that doesn't mean you can't find love.