As far as I can tell, the only thing you accomplish by neutering or spaying an animal is population control. Neutering has never stopped any of my dogs or cats from peeing/spraying to mark their territory or leave messages (pee-mail, as we call it here). Nor has it prevented them from having an active (if ineffectual) sex life. The current canine crew is an equal-opportunity bunch. Meg humps her brothers and her mother, Berry humps all his siblings, his mother, and Polly, and so on. I've been told that the humping is a power thing, but it seems to me they do it for fun. And Berry and Teddy often enjoy (they're certainly smiling while doing it) what was called "69" when I was in high school. Berry does the -um, mouth work - while Teddy does his humping. It makes me laugh. Mr. Parker calls them ugly names that I won't repeat. My mom would catch them at it and exclaim, "Oh please, not before dinner!"
This photo is of our first Frenchie, Buster, whose toybox included a stuffed cat. The first thing Buster did after coming home from being neutered was hump that cat. And he continued doing it at every opportunity until the cat finally had to go to heaven.
What interests me about this is that Buster and the stuffed cat always did it missionary style, not doggy style.