An escape valve is:

Something that allows one to relieve mental stress and tension in a harmless way.

A sexual escape value is:

Something that allows one to relieve mental sexual stress and tension in a harmless way.

Now, be careful.  This is kind of tricky.

A sexual escape valve is not something you do to relieve mental sexual stress and tension in a harmless way.  A sexual escape value is a gift.  A sexual escape valve never harms anyone, including oneself.  A sexual escape value does not require one to sin.  A sexual escape valve is a chatatophic reflex.