is there any graceful way to turn a chick down for sex without insulting her?

Yes. You can tell her that you are in a steady relationship with your SO. Or if you are married, you can state that fact. You can say "I don't swing that way." Or you could say "I'm not interested in YOU/a relationship at this time in my life." Any of those said in a nice, calm, friendly tone would work.

As to your Additional Details, you are right - guys are never taught to say "No" when a girl approaches them, and this sometimes works against them. Girls don't have this issue because they are TAUGHT to say no, to increase their market value.

From when they are very young, we tell girls that all that boys want from them is sex, or that boys would do anything to get in their pants, etc, thereby reinforcing stereotypical ideas about boys/men in their minds.

They grow up thinking that men are always looking out for opportunities to have sex with them, so they wouldn't say no to them.

Just say something along the lines of "Sorry, I'm in a committed relationship." or "Sorry, I've got an appointment to go to tomorrow morning."

Just say that you want to have sex with someone you love.

If she is not too ugly/ disease ridden, then may I ask if you are gay?

If neither of the above condition exist, just tell her that she is too good 4 you or that you are HIV positive.

Just tell her you have a girlfriend you're very in love with. Most girls will be affected by that and back off.

Hypotheticall yes reality no!

Whisper in her ear: "I have herpes."