I think, based on the few times I’ve had sexual disagreements with my partner, it’s due to insecurities leading to one or the other making assumptions and feeling like they can’t explain themselves and be heard in good faith.

It’s difficult to bring up issues you’re having with sex because it can feel like (to your partner, or to you if they’re telling you about it) like the most vulnerable, personal, loving act you can do is “not good enough”. And people usually already have insecurities about themselves not being good enough so when they hear (good faith and gentle) suggestions that “maybe us doing this differently would be better” or “I’m not getting everything I want out of sex with you” they don’t hear “I love you and I believe in our capability together for amazing sex so I’m telling you this so we can become better at it together” but instead hear “you’re not good enough”. And that makes them close off and shut down out of defensiveness.

That’s why you really need to have those awkward conversations when the issue first arises. Not let it simmer. Because no matter how awkward it is, if you love each other and trust each other, and want your partner genuinely to be happy, you can work it out and get past those knee jerk reactions of defensiveness.