In life of a married couple, sex is not a REWARD, nor a BONUS OR SALARY. But it is the covenant right at it was written in the Bible and responsibility of both partner.
Automatically your body belongs to your partner, according to:1corin7:3-4.
vs3 A husband should fulfill his obligation to his wife, and a wife should do the same for her husband.
vs4 A wife does not have authority over her own body, but her husband does. In the same way, a husband doesn’t have authority over his own body, but his wife does.
However, sex must be enjoyed by the two partners, relationship grows when SEX is served from both lover Hence,If you say thank you to each other after sex, it is a respect and courtesy for each other. But there is no hard rule or law that says you must say" Thank you after Sex".