I don't know how unusual it is, but a lot of mammalian sexual behaviour is
the result of primary rather than secondary sexual development (when the
gonads release sex hormones).
Primary development consists of genderisation of a particular region of the
brain - and determines the fundamental TYPE of behaviour engaged in .....
girlieness or boylieness.
In humans, genderisation for the opposite sex causes homosexuality.

when they neuter the cat, they only take out his testicles, he still has his
penis.

Most "dry humping' is often for domination purposes, not sexual..which is
why you see male cats doing it other male cats.  It is normal.. some cats
just feel the need.  I wouldn't worry about it.

It's totally normal. I asked my vet about this when my neutered male cats were
doing the same thing. One liked to hump my robe, another liked to hump a
stuffed animal, yet another would hump the other neutered male cat in a
domination ritual. If he seems to be enjoying it, just let him be. A friend of
mine had a female spayed dog. When friends came over she'd bring out this
stuffed toy cat and throw it into the middle of the room. This tiny terrier
would then have her way with the stuffed cat. It was pretty funny.