A childhood belief about reproduction

I was told the "facts" of life by a mother who believed that if I was told about sex, I would immediately take up the hobby (at the ripe old of 9, if I remember right) so my introduction to the subject was sketchy at best. Until I was well into high school, I believed that, just as a woman ovulated every so many days, a man only got an erection once every so many days as well. So, I did the math: if I ovulated once every, say, 30 days, and my future husband (heaven forfend I have premarital sex!) would get an erection once every 20 days, our "paths would cross" every 60 days; ergo, we could only have sex once every three months and each intercourse would result in pregnancy.