You don't indicate whether you are a man or a woman. If you are man, then obviously you are loosing body fluids when you come, which explains the thirst. Plus if you have a vigorous sexual session, you are burning energy, increasing your heart rate, and getting really worked up which also causes thirst.

If you are a woman, then believe it or not, you are also loosing fluids when you become highly aroused and this can cause thirst. The same thing applies for you as a man when it comes to vigorous sex making you thirsty. And if you lucky enough to have squirting orgasms, then you *really* need to stay hydrated. I have them and find that I have to stop and take a swig of water or some other liquid during sexual activities with my husband. I actually put a bottle at my bedside when we are planning a sexual session, because I know I will need it before, during and after

Sometimes but not always.

If I'm not thirsty then the sex wasn't good enough!