My answer to both those questions would be a solid, “No.”

Romantasy does not REQUIRE sex scenes. The justification for the deluge of sex scenes in the contemporary Romantasy wave is that these stories are Romance, but Romance itself doesn’t require sex scenes. The film My Girlfriend's Boyfriend is a Romantic Drama (i.e. another form of Romance), but it has no sex scenes, and sex is only referenced in a few mild jokes. Pretty sure the leads don’t even have premarital sex. Despite this fact, the film is still a Romance. It's plot is drivne by the romantic relationship between the main couple.

However, not having graphic sex does not immediately equal “clean”. You are absolutely right - it’s hard to really say a story is “clean” when it is loaded with other mature context. Warhammer 40K is relatively low on sex scenes, but I wouldn't call most of those books “clean” Science Fiction.

The reason people misuse “clean” in this way is a reaction to graphic sexual content being used as a marketing tool. We used to refer to such sexual content as “dirty” (like that episode of Friends where Joey finds Rachel’s “dirty book”, or how the film Saving Helen had the minister character joke about watching “dirty movies”). Contrasting that with “clean” made sense. Now, though, “dirty” content has rebranded to “spice”. “Clean” is not the antithesis of “spicy”, so now the comparison makes less sense.