Don't wait for the proposition before replying to the first bit. It's extra-embarassing to get to shoot your shot after broaching the subject only to be shut-down.
Girls are even worse about this, and they'll still ask you out and call you moron/must-be-gay-for-being-uninterested even if you reply to that first bit with "well, I like this other girl..." or "I tried that with ..."
Oh, and a girl you like but had no clue liked you also running from the classroom crying when she hears you asked another girl out is just ... absolute joy /s, let me tell you.
EDIT: "girls" was intentional; This would rank highly among behaviors that I personally draw the lines between girls/women and boys/men. In school, I worried about myself, because I never liked the more extreme/immature "girly" behaviors like so. Can't say it was a negative to end-up fixated on 25+, or at least mature-acting, women early-on though.
Sure she's not. Are you really that ignorant?
Short of the immediate physical violence that is FAR from out of the question from girls, do you write off having her family, your own family, thugs, your peers and authority-figures sicc'ed on you for made-up bullshit as nothing at all? Even just having "he made her cry" or "he's afraid of a girl" reach the wrong ears or big-enough audience can cause all-sorts of fallout.
It's amazing how much y'all weaponize the things "we don't do"(but statistics say otherwise) as an excuse to ignore all sorts of malice.