Natural contraception is safe but unreliable. why

I'm gonna use alterate names for things so it doesn't get automatically removed...

I'm assumimng this is talking about pull out or natural family planning (NFP) where you only hanky panky durring times of the month where the woman is least likely to be fertile.

NFP: Safe because it doesn't interfere with hormones or any of the body's functions. It relies on working around them. However, it's not reliable because the body isn't a clock. The egg can be released at different times from month to month and swimmers can live for up to 6 days.

Pull out: Safe because, again, it doesn't interfere with hormones. Unreliable because pre-___ can contain swimmers and it only takes one to fertalize. Men don't always time it right.