Yes it is. Apparently, FAM is very popular among religious Jewish women, as they can’t use some other birth control methods.
Some women hold onto the fear that using FAM will decrease the days they can have intercourse. But actually, using FAM will allow you to know when you are fertile and when you are not. And what you choose to do with that information is completely up to you.
There are women that wish to prevent pregnancy so they avoid intercourse during their fertile days. While others use other birth control methods such as diaphragm and spermicide.
The choice during those days is usually affected by your religious stream and your rabbi /spiritual guide’s advice.