I honestly don't recall that scene of course I need to go gouge my eyes out nude chibi-usa I don't need to see
It's a scene from the opening theme song that is meant to be a homage to the Utena opening theme.
But it really wasn't implied heavily though . (infact I didn't think they had sex) they kissed in the black moon arc . But in comparison in the manga it was toned down. What is weird they left the incest kiss ?
They showed their bodies as silhouettes lying down in Mamoru's bed while they kiss and rose petals fly by which is a very sexual symbol in a lot of anime. There was also a similar scene in s3 when Usagi and Mamoru make up after having their fight. Anyway, I'm not sure why Crystal wasn't airing on late night TV to begin with as it's clear Crystal is being aimed at a more older audience than the original 90s anime was.