Is this normal behavior for dogs?

My dog, Cutie is on he period. I know I know, she was supposed get spayed today but the vet there would be more risks if we do it now and my Grandma forced us to reshedule because she was scared something wrong would happen during the surgery. But anyway we have a male dog, Dexter he's neutered, he won't even try to go after her. But the exact oppisite, every time Cutie would try to make him hump her but Dexter will bare his teeth and act like he's gonna fight her. Is that normal cause I've heard that male dogs will do anything to get to a female dog when its in heat. These are my first dogs so I don't really understand this behavior. So is it normal? Or is there something wrong with Dexter?