I've said "no" Plenty of times.
I've (/knock on wood) never cheated on a girlfriend before - despite ample opportunity - that's the main source of my "no"'s, probably.
I started bartending at age 18, so come-ons were a regular thing. For those times I wasn't committed to another - some nights, I just wasn't up for it, so I'd make an excuse.
I learned my lesson early, and from about age 23-30 learned to try and avoid the extra-clingy chicks.... plenty of "no"'s there.
Also said "no" to all the guys that have approached me in my day (I'm still sorry about that, Juicy
)
Since 30, it hasn't been as much of an issue (been with the same gal for the last 3 years - and I'm getting that "I've over 30 and content gut"), but I still find myself saying "no... but thanks" at least a couple times a year...