A guy penetrating a female in the rear? GREAT!
A guy penetrating a male in the rear? Ooohh YUCK!
That kind of thing...
The fact remains that the goal of a species is to survive. Reproduction is sometimes harmful to the species' survival, something that cannot be denied, so your notion that homosexuality is not natural because reproduction is a biological imperative is complete bunk. Not reproducing may very well be a biological imperative, thus your statement that reproduction is always a biological imperative is patently false.
"Give me your tired, your poor, your huddled masses yearning to breathe free."
"When we live authentically we create an opportunity for others to walk out of their dark prisons of pretend into freedom."
First, it is masculine to penetrate and feminine to be penetrated. You see this materialise when a lady straps on and penetrates her make sex partner. This is called gay. Even though the action is sexual intercourse between a man and a woman some people see this as homosexual.
Second, for a male to penetrate another male would make the bottom feminine and the top masculine thusthe average guy views a top male as somebody who "feminizes" and objectifies males.
Men are less sexually objectified in our culture and it disturbs them deeply to think that they are objectified.
Women are sexual objects in our culture, not men.