I am new, as of 2018, to yaoi and there are a few things I feel like might be missing in translation in virtually every yaoi I've read, and one of them is the "sore hips" phenomenon....
Does anyone know what they mean by this exactly?

I am a female and have had lots and lots of sex over the years (including butt sex), haha, but I never had sore hips from sex exactly. Are they maybe referring to the butt, but don't want to say that? Is the word for hips in Japanese the same as butt? What are they talking about?! It's driving me nuts because I keep reading it in almost every yaoi I read. Why are there concepts like this that are almost totally ubiquitous and homogenized throughout yaoi manga?

Well, in japanese they use word 腰　which has few meanings such as (lower) back, waist, hips (you may refer to this jp-en dictionary entry for the full picture https://kotobank.jp/jeword/%E8%85%B0 )
As for why translators prefer to use 'hips' in this context...I honestly have no idea (lol).