The 'official' interpretation of the experts here would be a display of
dominance behavior. The fact is there are some cats that are flat out, no
doubt about it - poofters. Yours seem to have solidified into definable
roles as pitcher and catcher.

I had a litter of 6 kittens a year ago - 5 males, 1 female. One of the males
seemed intent on delivering oral favors to the other males, totally ignoring
the female. This too was described as dominance play. That may have sufficed
as an explanation until one considered that this particular cat was
dominated by the Alpha male of the group through the usual pecking order
displays. He also never seemed interested in assuming any of the other
traits one would expect of a male bidding for dominance. The only thing
obvious was a taste for his brothers' mating tackle in his mouth.

Yes, this is very common among animals--a large part is dominance--but do a
google search on 'gay animal behavior' and you will see that this is very
common in animal nature.

Take care,
Jen