What do you do if you're married and another girl kisses you?

I don't know what to do to stop it happening again. I wasn't on for it but after a while I gave in and kissed so as not to make a scene and embarr her. I need the other student's support and it could [almost certainly] make things harder and turn the girl aganst me if I turned her down flat. I don't need the other students ostracising me...... I did say no [gently but consistently] when she slipped the hand but how does one avoid this happening again?...... It's only once but she said that she's felt this way for a long time and if you could have felt the pent up need in her then you'd know that she's serious. Drunk, but serious.