I too, am concerned for the teeming masses of girls who continuously flock to me like lustful pigeons drawn to my proverbial bread crumb of manliness.

Jokes aside, the more serious any advance seems to be, the more serious and sincere I will be about handling it honestly.
If some drunk girl I barely even know is all up on me at a party trying to get laid or something like that, I don't feel I need to take the time to sit her down and bear my soul to her in explaining why I won't engage in sexytimes. Any excuse should do.
However, if someone was to appear to be honest and earnest in their advances, then I would certainly take the time to tell them the whole truth so was to avoid any wrongly bruised feelings or egos.