Weird mating habits?

My dog seems to be infatuated with my cat. Now, it's a pretty small dog, and my cat is fairly big (20 lbs). I haven't gotten them spade or neutered, yet. Well, basically, my dog keeps trying to have with my cat. I'm wondering, is it possible for him to get my cat pregnant? Like, can that happen, giving birth to a cat/dog type thing? I don't want any more kittens, or puppies, or any mix of them.