I think it really depends on their reaction. It’s perfectly fine to not want to have sex with someone on their period. Not everybody is going to be comfortable seeing blood, let alone getting it on their dick and/or fingers.
What registers for me as a red flag is if a guy treats you like you’re disgusting for having a period or talking about it in any way. Periods are natural but they aren’t the most appealing thing, so I don’t blame them if they’re a bit antsy around it. That being said, if they make you feel gross, ashamed, or like you’re not worth their time because you’re body is going through such a natural thing, that’s absolutely not ok. You should still be treated with respect and you should still feel loved. Just because they are not comfortable having period sex, their opinion and treatment of you should not change while you’re on your period.