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.