How can I stop "sex hair" from happening?

Hmmmmm.
This is a sticky situation, or can be. I would say that mutual aftersex grooming might help. Either that or try to be on top from time to time.

Put them in pig tails,,,,,,,,,

I’m guessing stop having sex is not an option?

I can honestly say I’ve never had this problem. Messy hair, sure, but nothing a brush (or fingers) can’t fix in a couple of seconds.

Maybe stop wildly thrashing your head against the bed?

A satin pillowcase might help.

You could wear a surgical hat, or shave your head.

I guess I have been having the wrong kind of sex all these years.

You could just have really lousy sex so you don’t move around at all. It’s nature. Go with it.

How about a full body gimp suit?

Satin pillow? Good idea.

Only me? I do tend to be squirmy. I’ve never had good sex that didn’t end of with me having seriously tangled hair.

I am on top at least ½ the time.

Since this is in general, I won’t suggest what first came to mind!