I would make a cuck/bondage/zoo scenario with a hetero couple and a male dog (or multiple, why not), where he wants to see her get licked by the dog and she accepts on condition that he licks the dog’s sheath first.
Things slowly build up with a back and forth of control and playful challenges (not all involving animals). The man slowly enjoys more submissive acts, and ends up wearing a chastity cage midway through.
It escalates to him wanting her to get mounted and knotted, and she agrees in exchange of him drinking canine semen out of her and cleaning her up with her mouth… as well as getting the same treatment. In fact, for the next (multiple?) month(s) he will not be allowed to cum other than hands-free when fucked by dogs.
We see all the challenges and acts, with some intimacy, and some movie magic to make the humans-only bits fun but not lingering. Last few scenes are of him getting pounded by a few different dogs in a bunch of settings/places; till the last one where a much larger than average shepherd takes him roughly and we finally see ropes shooting from his dick cage.
She calls him a good boy and removes his collar, leash, and cage but he doesn’t seem sure… "to be continued"