Why do boys find it disgusting when girls are on their period?

The other day on a group chat one of the girls randomly stated how she wasn't feeling well because she was on her period, and immediately a lot of the boys started to go " EWWWWWW PERIOD" and " UGH THEY ARE DISGUSTING" and these boys are like 17-18 years of age. I know that period is known as 'dirty blood' but it's not like these girls are bleeding on them or anything, usually no one knows that a girl is on her period. Also recently I heard another boy talk about how it's weird to think about cuddling with his girlfriend when she is on her period, I was shocked because if that's his girl then he shouldn't be weirded out. All I'm saying is that it is something that them girls can't control and some of them go through a lot of pain (cramps) during it, the last thing they need is a bunch of immature boys telling them how disgusting it is.
Why do boys find it disgusting when girls are on their period?
Post Opinion