It's pure pecking order behavior, except you don't seem to want to accept
that.

Seems to me like you are not comfortable enough with your bi-tendancies yet.
So maybe ask your therapist "what gives".

why does it gross you out?  They do it because they are cats, and SOMEONE
has to be in charge.  Being neutered it has nothing to do with sexuality.
It is pure politics.

I usually stop it in my household when my two strapping boys start (I have
one older cat but she's MUCH older and is now the house elder, and has no
interest in being in charge)  Its usually Ollie (the older at 9) on top of
Jack (who is 4) because Jack likes to think he's in charge (mostly cause he
has me wrapped around his little finger)

However, in my household, I want to be the one in charge, which is why I
stop it. and I'll pin aka scruff them when they are asserting dominance
around me.  Makes it easier on me when I need to groom them, or treat them,
etc.

Six cats here too.  the last four were done before they were sexually
mature, so they have no idea what sex is.

Be kind to them.  Despite "common" wisdom by people who can't
distinguish between what is true and what they want to be true,
homosexual behavior is quite common in animals.  (I'm a professional
naturalist.)  And despite all too common prejudice, it's harmless.

If it grosses you out, and you still want to own cats, you do have an
issue to work out.  (And despite common prejudice, homophobia does NOT
mean you are homosexual and in denial) but yelling and pushing them
apart is not fair to the cats.  Be kind to them, and be kind to
yourself.  Please.