At the end of the day, you can fuck other people in game. It might not be "exciting" like a usual porn game, but the content still exists.
I think you're having trouble with this just because it isn't as explicit as usual. You can buy and sell slaves, aliens can "impregnate" people and have their babies burst out of them. The npcs regularly jokes about another npc's fine ass. You can have sex with your party members, there's prostitution, you can kill children in horrible ways, you can fish up porn books, etc, etc. The game might be pretty humorous and vague so it's easy to think of it as fairly harmless, but it certainly has some questionable content that I don't believe would be suitable for children lol. The amount of flexibility alone makes it kind of adult in nature by default. Is it as "explicit" and "adult" as most adult games here, absolutely not. But acting like it doesn't qualify as adult is wrong too. It's greyish for sure, but I've seen much worse here.
Some people maybe don't know how much + and custom have added. I don't think the sex stuff used to be as fleshed out, I think base Elona and early + didn't allow you to have sex with your party members if I remember right. You look me in the eyes and tell me that sex description at the bottom of the image isn't lewd.
I am surprised people haven't modded more porn into the game, must be a pain to mod. It did get me to make my own nude sprites for it though, lol.