I recall actually reading some blurb about insects....kinda real life version of the fictional salacious TV doc in the above Python sketch.
Male bed bugs will "play with each other" (give each other hand jobs?) and will become covered in each other's semen.
And then when one finally does find a female to mate with...she gets not only his semen, but that of his male friends.
And in the same article they went on to talk about some species of wasp...the males will kill flies and then will offer the flies as expensive gifts to females in order to mate. So human males are the only males who pay for sex. But some males of this wasp will shake thier asses to LOOK female ... to get other males to offer them the flies...so they can then steal the flies to...use them to buy sex from females.
But the two above examples are both of use of homosexual behavior to actually serve heterosexuality.
Returning to the vertebrates...I have read that some small percentage of female penguins pair-bond with other females. They even lay infertile eggs together. So that would be true lesbianism in the animal kingdom.