Just why?!

Because they don't want to have sex with you and are using a headache as an excuse to deny you politely

lmao i really hope this post is a joke

If they really do have a headache then having sex and being in pain is not what most people want. Here how about you have a headache and build me a shelf.

Maybe you're the one that caused their headache

Cause there's a difference between can't and don't fricking want to

Have you ever had a headache?

I always thought that was strange. In my experience, sex CURES headaches.