[Potentially NSFW] What is so upsetting about being hit on or flirted with by someone of the same sex?

I really struggle to understand this, particularly because I have nothing to compare it to. However, assuming this person is not going to rape you, what is the harm in being pursued by someone of the same sex? Or, I suppose if you’re homosexual, someone of the opposite sex.
Is it more disturbing than meeting a woman at the bar and learning that she’s wearing a padded bra, or that she’s saving her virginity for marriage, or that she has some unforgivable quirk… let’s say a bedroom overloaded with stuffed teddy bears? Maybe you’re in a very dimly lit room and when you step into the light you realize the cute person you were just talking to is not nearly as attractive as you first thought.
Why is it perceived as a deception or sometimes perceived as more ominous than a deception, simply because a person has sex organs that you prefer not to have sex with? There are countless reasons that a flirtation never makes it to the bedroom, so why is this issue more significant than any other reason that two people may not be compatible?

Is it any more of a deception than not telling someone that you look better in subdued lighting, or that those aren’t really your breasts, or that you have a fascination with stuffed animals, or that you have zero intention of sleeping with the person you’re flirting with?