It is not advisable to have sex at all up to two days after the hair transplant procedure because sexual activity increases blood flow and blood pressure.  Increased blood pressure can push the grafts out.
On the third (3rd) day you can have sex until the tenth (10th) day but please be careful to make sure that during sexual activity that your face is never face down.
Also make sure that your partner never touches the transplanted area.
Lastly, make sure that you do not push yourself physically during sex or that you make minimum efforts to move.
After two (2) weeks you are free to do as you wish.