I'm far from being an expert but IMHO the chances that your neutered male dog will try to "mate" or "hump" your young cat are minimal. Often dogs who try to hump a cat for domination purposes get a real surprise when the cat turns on them - their claws and teeth are really sharp. Depending on the cat's personality, she might just dig her claws into him if he tries anything. Even if the dog does try to hump the cat, there's no real harm done. Personally, I'd get the cat spayed asap. I have a kitten who is in heat too and her meowing is incredibly loud. She has an app't to be spayed next week. None of our cats or dogs go near her, not because she's aggressive but because her meowing is SO loud - kidding but not about getting your girl spayed = )
Has your dog tried to hu mp your cat? If he has, it isn't to mate with her. It'd be more of a dominance thing. Yes, fixed animals can show dominance and different species can't/don't really feel the need to mate. If he tries to hu mp your cat, make a noise but don't look at him or mention his name, just clap your hands real loud and make a noise. Your dog 'wanting to apparently mate with your cat' is more of a behavioral issue. I bet the folks on the dog forum would be able to help even more as the real problem is the dog, not the cat. My Belgian Shepherd would put my cat's head in her mouth during play time. No hum ping....
Good Luck!