get it because its like 69

First off you gotta understand that to a degree it's adjacent to a hyper kink, or flows from it in like a divine emanation sense. Hyper starts off (or, can) as a fixation on the body parts in question themselves, but as you gradually horseradish-effect yourself it becomes a kink for size, scale, and numbers. You relate long strings of numerals to horny shit enough that you start getting horny for the numbers themselves, you compare vastly mismatched sizes enough that you get turned on by one thing being many times bigger than another thing in a purely abstract sense. A room, or series of rooms, will definitionally almost always be larger than anything occupying their interior, and thus the spatial extent of these constructions (which is often anomalously great or even infinite) becomes read by a sufficiently distorted brain as implicitly erotic.

Secondly, I think my personal background is tangentially relevant. I grew up pretty lonely and what little '''sexual experience''' I had in my formative years came in the form of online roleplay with friends, most of them also transgender women. Description of physical body interactions per se was often minimal, owing to a combination of dysphoria, presenting to each other as weird esoteric nonhuman OCs, and inexperience. There are a limited number of ways as a teenager to type 'I put my penis inside of one of your orifices and move it back and forth' and remain interesting. The engaging part of 'sex', then, became not necessarily the literal sex act itself but the dynamics, kinks, interplays of personality. Understanding what someone is into, and convincing someone to do what you like, is an intimacy tied to sex disconnected from penetrative or bodily action. A sapient complex space that can be comprehended, explored, interacted with and learned is effectively a partner for this kind of intimate interplay of minds and concepts, and indeed as I've written about previously can change the smaller party to fit the bigger party's nature/needs/desires.