When do you start letting others know to back off from your husband without losing your pride and self worth?

we have been married for 12 years and never had issues about affairs,modesty aside we have both shared confidences about how others have expressed their attraction for us and these people are aware that we are married,however he went out of the country to attend his class reunion and told me how he got so drunk one night and this one girl and him started kissing in front of his schoolmates, i was livid but he did promise to stay away from heavy drinking unless i was physically present with him,i "trapped" the girl who did it by pretending to be him on his facebook because he said he passed out and since there were missing links to his story i had to find out for myself and sure enough i did i never swore at the girl i was cool enough to let her know at the end of the chat that it was not my hubby but it was the wife talking to her on facebook don't know if i scared her off or embarrassed her because in that instant she deleted her name from my husband's facebook i have always trusted my husband until this started to happen if he stays with me it's because he wants to and not because i'm constantly guarding him so where do i draw the line?