I've had sex with a regular WG when on her period. TBH it doesn't bother me too much with someone I've known for years. She will always tell me she's on. When you've booked and prepaid a hotel with no cancellation available, you kinda have to go ahead with the booking, especially if you have no plan B.
It would be totally gross with a new WG. many WGs in parlours work when on, by using a sponge. I don't like the feel of a sponge.