I have heard of sexual encounters in which the partners name is not known, but but how I read this is that in 25% of gayses, they can FINISH an encounter without knowing sex of the other person.
I think “known” in this context means “known to CDC researchers”. Even among those who are afflicted with homoerotic desires there are those who do not like to broadcast their sexual proclivities (yes, I know, given the “Gay Pride” movement that is not obvious, but it is true, nonetheless) and who thus might decline to tell even their physicians the details.
That was then, this is now. Where a dude with sausage and two veg might have decided they are a woman