Both my boy cats are neutered.
I dithered for ages, especially as they are pretty much house cats and never go beyond our backyard - plus I felt peculiarly guilty for some bizarre reason (reverse feminism?).
I ignored the increasing incidences of little feline cocks popping out and mini-spraying (or "scenting" as bowdlerising DH called it), but the final straw came when DH got out of bed one morning after a lie-in, visibly shaking and pointing to his forehead, going "smell this! smell this!"...
...one of the little chaps had "scented" his head
and off we were to the vets the next week