I agree that sex scenes like violent scenes that develop the characters or characterisation shouldn’t be shy away from. I think the main issues with those kind of scenes is when they turn into the mechanics of what goes where and who does what. The scenes I have read where it have really works were when the reader is in the head of the character, even if they are an onlooker they would feel fear, disgust, maybe shame at not helping, basically how do they react to what they witness. For example, Gabriel Tallent’s My Absolute Darling doesn’t shy away from sex and violence but for me those scene always revealed something about the characters, their inner psyche and the inner workings of their relationships.
I hope this helps.