What Does It Mean When A Woman Says Thank You After Sex..?

It depends on the situation. Maybe she's being sarcastic. Maybe she's being honest in saying she was desperately in need of a fuck, any fuck. More likely, she's simply being genuine in her feelings. I remember the first time I did anal with a girlfriend, she thanked me when it was over for being so gentle and considerate. I was glad when she said this because I'd been worried about taking her this way.
Anyway, the question seems kind of "me-centered." Furthermore, guys should be able to say thank you too. I remember one chick who turned me on so much that I came as soon as she touched me. I wasn't that experienced and felt so humiliated i was ready to cry. Some chicks would have been laughed at me or been angry. She simply smiled and said I would last longer the next time we did it that day, and even longer the time after that. For sure she deserved a thank you.
 
to me it means they needed fucking, and I successfully got the job done. my wife of 42 years sometimes still thanks me but now it means not just sex but for everything.