I know this is completely forbidden in humans but is it sinful to neuter pets?
Nope. It’s more comparable to pruning a fruit tree than it is to destroying human sexual faculties.
It is actually a very good idea! I love cats, but we don’t need any more of them.
I volunteer at a low-cost spay/neuter clinic for cats. It’s my main charitable activity.
God gave man domain over the earth including the animals, so man can decide whether to allow life to foster in animals (animals in their natural state) or prevent new life (neutering animals). The animals are here to serve us, just as we humans are here to serve God.
Absolutely not sinful.