Q4women: do you ever turn down sex with your SO?

When your SO initiates sex, is there ever a reason why you would I turn him down? Do you ever tell him that you're too tired or you have a headache?

I understand that there are reasons such as feeling genuinely unwell when you are likely to deny him, but I'd expect in that case he would notice you were out of sorts and not try to initiate.

I never turn down sex with my SO. And I try to be as enthusiastic as possible every time but obviously there are times where I'm super tired and I kind of just let him do his thing and then roll over. I wonder if that is better than no sex at all?