>masturbating always helps
When writing erotica, fuck yeah it does.
Some more:
Maybe the cult leader followed a pilgrimage for some lame god who's all about order and control and "missionary for the sole purpose of procreation," but the main characters are following the pilgrimage of a goddess of chaos and pleasure and embracing the unknown and new experiences. The cultist has a rod of mind control or something, and he can make anyone who questions him kill themselves or serve him faithfully, w/e, at will, but supposedly there's a relic that can counteract the rod's effects and that's what the MC's are after.
Or pick something else. Still just brainstorming. You're free to use any of this or ditch it all, idc. I'm just getting off.
Maybe go the way of the minotaur/labyrinth, but that one's been overdone, even in erotica.
You could have a succubus/incubus with tentacles.
Impregnation is a fetish a lot of people have. Maybe girl (b) gets preggo from some weird creature and hatches a clutch of eggs, lel.
While camping they could take turns keeping watch, but one person wakes up to find the person on watch masturbating, because his/her libido is all flared up from all the stimulation on the adventure. Everyone likes getting caught masturbating.
Some temple could have an underwater passage filled with mermaids. They're like electric eel mermaids tho, so every time they touch a swimmer, it sends a jolt straight to their genitals. If you've never seen a guy cum in water, you should go watch a video of it right now, because describing that in writing could be fun. Ofc there's the danger of drowning the whole time too, holding breath drama, before he gets the key or w/e.
They could get mugged/raped by highwaymen. Or maybe they use what they've learned to distract the highwaymen while whichever one of them has combat experience gets into position.
Walking in on bathing scenes is always fun, and would let you continue to build the romantic/sexual tension.