Couple of times...
1. Girl tried to get fresh with me at a school where I helped out with the computers. She was at my house and gave me a (joking) ultimatum: sex or she leaves. I said I had a g/f so it had to be no.
Even undoing her jeans didn't help.
2. A 50+ woman who I'd met as part of a group a few times chatted me up on IRC and was trying to convince me to have sex with her. This was while she was in another chat window with my girlfriend saying what a nice person I was. I was 24 at the time so her chances were nil regardless of my relationship status. She ended up with a guy about my age anyway.
BTW, once you have kids you'll be saying "no" a lot more... or you won't need to because you'll both be so buggered you won't even bother to ask each other...