It won't get sexual content for a while, but that was the main reason. The 18+ is also for including violence, death, drug usage, and harsh language. It mainly depends on the route you are playing though, if you are too sensible to these subjects I reccomend you to ONLY play Derrek's route, if not then play the remaining paths.
Derrek, at some point, will get a nsfw scene. What I meant was that the themes portrayed in his route will not be as "heavy" as the ones in the remaining routes. Thats why I encourage people to play Derrek's route before any other due to its lighthearted nature.
Apologies for misscommunication in previous reply.