They’re being turned on by getting the strings on their third eyes all tangled up.
If the body part is clearly nonhuman and lacks an analog (or is animal ears) and just having that part touched sexually excites the persion in possession of said body part, then it is this trope. (That, and seriously unique erogenous zones.)
What should we do?Let's go back.Help us out first!!Ahn! ♥Uwaa~!
Don't move there~!!Wait, Koishi, we're somehow entwined together...Onee-cha... Ahn! ♥Say, you two should also do this.W, we're the home owners, so it's okay!Wait... Geez, why are we in this situation!?PromiseWha- What the-!?
What's going on!?Doesn't seem there's much of a punchline.Eh- Zero reaction?
And further, they don't think anything of it!?