A/B/O 'verse, Hungary is a rare female Alpha in a relationship with Omega!Prussia. However, when he goes into heat due to her own biology she is unable to give him what his body needs - having sex with him takes the edge off, but it doesn't satisfy his heats in the way being fucked by a male Alpha does. He tries to convince her it's okay (he's used to riding them out), but it's acutely frustrating to her to be physically unable to follow her instincts as an Alpha, and she gets angry at the whole situation and it puts a lot of strain on their relationship

Bonus: She sometimes enlists another Alpha to fuck him through it while she watches, even though it hurts her and turns her on in equal measure.