Are you absolutely sure you and your partner not changed default content filtering preferances on Steam?

Because Steam literally have settings in "Home > Account -> Store preferences -> Mature Content Preferences" with following options:

> Some Nudity or Sexual Content. Games or content that contain some nudity or sexual themes, but not as the primary focus.

This is where basically 99.9% of your normal games belong even if they have sex in them.

> Frequent Nudity or Sexual Content. Games or content that primarily feature nudity or sexual themes. By ticking this box you affirm that you are at least eighteen years old.

This is where all the sex VN slop belong.

> Adult Only Sexual Content. Games or content that are sexually explicit or graphic and are intended for adults only. By ticking this box you affirm that you are at least eighteen years old.

And this is porn.

I'm absolutely sure that "Adult Only Sexual Content" is disabled by default and pretty sure "Frequent Nudity or Sexual Content" disabled too.