As a man, I will say sex is not like a switch (on/off). There are many factors mostly in our heads that might cause us to avoid sex.
When we are young the switch never gets turned off. However, as we age and our partner's age there are little voices in our male heads that cause doubt about the switch coming on. So rather than find out for sure we might avoid sex altogether. The blue pill does not make us horny. First, we have to be aroused then it helps get and maintain an erection. However, if that little voice won't shut up it ain't gonna happen. Been there done all of this over the years.