it depends how it's prepared

No idea why some may be terrified. Just another day between the sheets, in my opinion.

Never bothered me , just get a towel to keep sheets clean , or do it in the shower

Sadly some of it may still be a Hang over from the beliefs of the Abrahamic religions, which regarded women as tainted, by Eves sin. And saw menstruation as Gods punishment, and proof that females are unclean. Fortunately very few sects still practice the old ritual of, confining women to a special womens house/prison, outside the village, during their period. But some of the attitude perhaps still persists in fundamentalist groups and spreads into the mainstream community.

That is probably not helped in strongly religious communities who try to keep the sexes apart, and ban anything to do with sexuality, from conversation, so that many men grow up thinking that the opposite sex, and anything to do with them, are alien and frightening anyway. Which helps to feed misogyny in religious communities, and which in turn feed the separation of the sexes, feeding more mistrust, and so on in a never ending cycle, so much so, that some religious communities even bar female animals from the houses. In part it may be all another thing that you can blame religion for.

Never bothered me. I have had sex many times with women during their period and got my "red wings" in my late 20s. It has been quite awhile as the women I have been with in the last couple of decades are well past that part of their lives.

"It can be disconcerting when you see the blood. It's like, OK, wow, your sexual organs are not just a place my penis can hang out." —Omari G., 31

What Guys Really Think About Your Period

[cosmopolitan.com]