i mean most sex games tend to make lust driven zombies you could go down a more feral route with the zeds instead of them spreading it thru biting maybe they drag people to their lust nests which have a spore that causes your mental abilitys to only show lust meaning they are zombies but their pure desires are mainly lust and fucking
you could make a few screens with zeds where they are sated but our debters could find themselfs with infected who are slowly turning they can choose to fuck them or hide if you hide it would be random if they find them but if their found after hiding the infected would drag them back to their nests in turn ending the game in a infected gameover screen
but if you let them use your body they might even be passive enough to let you choose how you mate them