Finn’s sexual awakening took place while he lived a crowded barracks amidst like a hundred other people, and Rey’s sexual awakening took place while she lived in an abandoned AT-AT miles away from anyone else, so Finn’s approach to masturbation is like “be still, be quiet, pretend you are still asleep and nothing is happening under this blanket, what could possibly be happening under this blanket? nothing because I’m asleep,” and Rey’s is like “time for screaming, it is scream time”
Which means they first time they combine their mutual sexual experience, it just stops halfway through so they can both be like, “UMMM SO I’M HAVING A GREAT TIME BUT WHAT ARE YOU DOING”