Answer:
Kinda-sorta. Looking at the pure biology, there's only a window of a few days during each period when a woman can get pregnant. So if a woman knows exactly where she's at, it's possible to avoid pregnancy simply by avoiding sex during those days.
Trouble is, bodies aren't clockwork. They fall out of synch a little for whatever reasons, and get up to all kinds of tricks and cheats.
And it's those departures from the norm that makes counting days and rhythm methods so unreliable.
"Safe" days aren't safe, only days of varying risk/chance of pregnancy. One out of five couples relying on counting days find themselves pregnant within one year.