It’s BS, but it does come from a twisted/uninformed interpretation of two facts…
1) the Rencho culture doesn’t have anything like marriage, and extramarital sex is very taboo in Keol (unless it’s some dude with a prostitute… double standards FTW!).
They do have committed long term relationships, but that’s strictly between the partners and have absolutely nothing to do with in-laws (which isn’t even a thing for them). Renchos tend to view human marriages as family politics, while Keol humans think of Renchos as… free spirits, to put it gently.
2) the majority of Renchos are pansexual. That’s where they get the geoduck thing. Back in the days, Keol humans thought you couldn’t be attracted to a woman unless you had a sausage, or to a man unless you had ladybits. Therefore, pansexual must mean the person must have both parts!
Even though THAT myth has been debunked for quite some time now, the unfortunate nickname has stuck.
Back to the tea, the “logic” is that if someone doesn’t care whether their partner is male or female, they must be really, REALLY desperate and completely unable to control themselves. Of course that’s not true at all, but… :\
A lot of this will be shown in the comic eventually! But the awesome folks at Patreon suggested putting it here for curious minds willing to brave the tl;dr. Since this isn’t heavy plot spoilers or anything, I figured it wouldn’t hurt.