lol, the same exact thing happened with my neutered male and female just prior to her being spayed. My female was a little floozy! She would stick her tail up and back right into him. My husband and I would joke that he didn't know what to do, but then I heard him crying and walked in the mud room and there they were!
I callee the vet and he said Bentley could still get excited even though he was neutered but of course she couldn't get pregnant. After that little indiscretion I kept him in a belly band and her in her little bloomers.