Oh dear. It's puzzling that this behaviour has started immediately after desexing ...

Tabasco was fine after his snip snip; sometimes I feel that Mini wasn't done properly because he still mounts Jasmine sometimes, but the vet said it's his way of showing dominance over her (but he doesn't do anything but mount, and look a little confused).

I'd think about taking him to the vet's for another opinion. Good luck.