A penis can also fit in a dogs ass, meat grinder, a arm pit etc.....
Sure, but none of this suggests that it is
objectively wrong to stick a penis in places other than a vagina. As such there is no "they must know its wrong" because it's only wrong
in your opinion and there is no reason to believe that your opinion, or mine, or anyone's is shared universally.
As such, you can certainly look at the practice of homosexuality and critique it (on objective grounds, like its tendency to spread disease or biblical prohibitions against it, and even on subjective grounds like the personal disgust you feel for the practice or the reasons you feel its immoral), but if you are going to pronounce it "wrong" then you are merely stating your subjective opinion and not an objective fact.
If it's not an objective fact, then there is no reason to believe that every homosexual somehow magically "knows" that it's "wrong", as their subjective opinions may (and usually do) differ.
It's rather like saying "The Aztecs must have known that human sacrifice is
wrong." You and I may agree on the morality and "wrongness" of sacrificing people to the sun god, but the Aztecs surely did believe that they were *right* to do so.