It's a very old Chinese saying, it came about when people saw monkeys "playing" and it was rude to call "grab balls" and since peaches looked like balls, it became "monkey stealing peaches". Classier than "grab them by the balls" don't you think?
What are the odds OP's avatar is actually a gender-bend/trap twin(s) penetrating each other with their demonic tails? Don't ask why I imagine it like that. It was a horrible experience. BAD ENDING! Ugh, it reminded of something nasty about magical girls.
You know, if someone with no knowledge of the context (like me) suddenly jumped in just because of the weird title, they would've thought that this is a bad shitpost. I suggest some fine quality Vanilla H-mango for the remedy. Read before you sleep to make it better.