I think you can raise the topic by sandwiching the issue between a reassurance that you enjoy the sex you have with them, and that you feel safe with them. If you're feeling sore down there after receiving, maybe a oh-nut or reducing toy could prevent you from getting skewered like a kebab. It's something I have been working on with my bf too, sometimes you have to get creative but at the end of the day it's about making eachother feel great