It's a thing that can happen. We call it a "Surprise Sex" or "Jun Jump On You". It can be triggered if Jun loves you and you didn't have sex with her for a few in-game days , and it will trigger when you go to sleep. If it's happening, you have to wait for her to be satisfied, or keep asking her to stop, or asking for you to take the lead instead of her.
It's not a bug. It's not a glitch. It's a feature.