Because of subliminal conditioning. Because we've been lied about "everything". Because we believe everything is subjective and relative and that there is no absolutes. (I'm using "We" in a general and loose sense.)
And because we like to think whether overtly or unintentionally in secret that we are brilliant, but really we know nothing but actually pretty dumb. Or not, if you like to think otherwise.
We have owners people, whether we like it or not. One can say a lot of words to give brain-orgasms, or write mounds of psychological thesis to mesmerise simple constitutions. It is foolish to consult those versed in the School of Psychology garb without good "discernment".
To begin figuring out the answer to your question, start by asking the right questions, unless you are looking for the right question to an answer you already made your mind on to see how things could work usually in abstract.
What is sex?
What is sexuality?
There are plenty of those that chose to take on theiasked as because of certain predisposition. Words have meaning. You can't say a creampuff also means roast beef. Well you can, but that don't make it so in truth.
Sir Clement Freud was Sigmund Freud's grandson.
If you don't believe in absolutes, why should it matter.
Less fancy. Is this a question that requests a philosophical, or enquiring about real reasons for the instances the original poster mentioned?