The first thing that comes to mind is to have Hitami(legally distinct!) spurn him in favor of a beast girl. Preferably one that he bullied. Maybe as a result of how he treated her, even. Hitami decides to comfort the victim over their mistreatment and then they tell him to fuck off NTR style.
For the sake of the deconstruction, the question to ask here is "Does propagating the species really justify the relationship?" She should step back from the relationship and force him to reconsider his prejudices and overcome his flaws. If it weren't for the 'last of their kind' thing, that would be a permanent break up, but they might get back together if the author wills it. If he refuses to grow, then humanity can go to hell.
Usually this sort of thing would be done from Hitami's perspective, though.
Alternatively, you can portray Hitami as a tragic victim here. She's willing to sacrifice her principles and her own body for the sake of the species, even if she hates the MC. This won't do well on SH since we like the lighthearted stuff more, but whatever. For bonus tragedy points, have both of them get murdered by beast kin in retaliation for something the MC did.
She could also go psycho and put him in a coma so she can harvest his sperm without dealing with his shit.
Catharsis is the relief/apathy one gets after experiencing strong emotions. It's why people like Tragedies and bad Romances even though you'd theoretically never want to read or experience those things.
