For all it´s big flaws, Steam is not the main problem here, they allow more leeway with content. For instance, in the second episode of "9-Nine", even the kiddy-friendly version, you have to be a dummy to not get that one girl you are pursueing is your Sis.
Steam are oxen when it comes to age-verification, which is the reason e.g. we Germans have a lot of hassle getting some stuff, but unlike Patreon allow more sexual stuff be mentioned in games from the get go and they give a flying fuck if you later use 18+ patches. Patreon today on the other hand tries very hard to outpuritan the puritans.