Yes and no. Prince of Suburbia is a thing. A game with breastfeeding in it. On steam. Thats why I don't get it, you know? :( Steam is being weird... But thanks for clearing up my confusion.^^ Also: I can activate several other adult games (their steam keys I mean) but not this one. Just as weird despite them being 3d and therefore counting more as "almost real porn" and thus "violation of terms and services" than this game does. :/ Why can't they leave your game tf alone, for christ sakes?!
Dunno about Prince of Suburbia or other games, or why their content was allowed. Perhaps Valve's compliance team overlooked it. Perhaps it was patched in after the game was approved. Perhaps Valve's policies about said content has changed over time. No matter the reason, I don't think it's worth risking Maid Mansion being taken down from Steam just to add a very minor bit of content. If you really want the removed scene on Steam, you can just move a "bpatch.rpa" file from the Itch downloadable version into the Steam version's files, and that'll restore the missing content.
As for the game keys, the only explanation that I can think of is that the other adult games you mention are distributing "Beta keys" rather than keys for standard retail copies of the game. Beta keys override things like region-locking, which would allow customers in Germany to own the game, but those kind of keys are definitely not supposed to be distributed to the public.