I suspect most people reading this book won't ever have had sex with three people in the bed at once, and may have a difficult time picturing it, extrapolating from their own twosome or onesome experience. I think a concrete grounding here, the first time they're all together, will keep me from having to spend a lot of words on it later, if I do it right. And it's plenty emotionally complex to have that later to add to the physical detail -- the chapter's really more about the emotions than the physical sex. So I think it's okay. Maybe.
Maybe I just need to wait until I have an entire draft before I can properly evaluate whether it reads weird, to have an entire chapter be one long sex scene.