Real sexual desire is considerably less obedient. It changes with stress, sleep, mood, health, relationships, medication and life circumstances. A man can love his partner, find her attractive and still have absolutely no desire for sex tonight.
But the myth of the “always ready” man creates a problem for both people. His partner may hear the rejection as “I’m no longer attractive to you.” The man himself may hear it as “Something is wrong with me because I’m supposed to want this.”
Suddenly, a perfectly ordinary “not tonight” requires a medical, psychological and romantic investigation. So I’m less interested in whether we consciously believe the stereotype than in whether we notice it in our own reaction when a man says no.