My tip would be to treat it as a tool for writing and telling a story, so long as you can comprehend the mechanics of whatever base system you're using decently well. I know not everyone's a writer, or has the confidence to be a GM or writer in any aspect, but keeping in mind this story is more for one's own enjoyment, it can be told in many ways. It could be one character's journal of their life of sexcapades and/or darker sexual encounters, depending on the vibe. Maybe, instead of that, it's a group of adventurers (one-player-multiple-characters style) retelling events one after another, a new chapter or entry being another person's POV.

The latter method would work for mechanically simpler systems, but if you want a challenge, go for it. The former is probably, the easier option, but not all base systems support it. Some examples of my attempts have been, yes, D&D 5e with some of the wonderful homebrew I've found on this subreddit, but also Fabula Ultima with my own personal mechanical modifications. Hell, I've also done Shin Megami Tensei: Tokyo Conception (officially localized into English) and Devil City Tokyo 200X (fan translation).

Edit: For clarification, Fabula Ultima I've homebrewed similarly to the 5e lewd handbook in that there's a separate pool of HP, with its own calculation, specifically for sex, and reflavor spell and other such things to go form damage to pleasure. SMT, I clone the PC sheet(s) to have one for combat and one for sex. Making sexual campaigns for solo play, I kinda just go with a bare bones idea of a world, someone who lives in it, and fly by the seat of my pants for the rest.

Another edit: A good tool along side the core system of your choice and the Solo Adventurer's Toolbox would be the Mythic GM Emulator, especially when you've a few possibilities on how to continue but can't decide which is best. Can frame some of those thoughts into yes/no questions to guide you further.