Sex is not a stress reliever. It is a form of most intimate communication between two people. It is an honest expression of innermost feeling in an atmosphere of trust and closeness.
It is incorrect to think that every time I am stressed I will go for sex. That will not eliminate the basic reason why you are stressed at the first place.
In fact, if you first learn to destress yourself and then allow natural intimacy (sex) you are handling both - sex and stress - in the correct way.
Mixing the two may vitiate the quality of both.