If you wait for the perfect moment, you might miss the best opportunities.
Instead, think of sex education as an ongoing conversation.
Sometimes, they guy would be cheating on them with some random girl that he met at work or at a bar, while at other times it would be someone the girl knew fairly well or it was even her friend.
Being scarred by prior disappointments in guys who cheated on them, many women are unable to enjoy their present relationship, even when everything seems to be great, because of that constant fear that sooner or later their boyfriend, or a husband will cheat on them, just like the others did in the past.
Nothing will replace the excitement of, and the satisfaction from, having new experiences with new female partners, and it would be pointless to try to change such a guy.
This desire for novelty and variety of dating and sexual partners is particularly strong among divorced men who come out of a long (unhappy / sexless) marriage.
By reinforcing and supplementing what your teen learns in school, you can set the stage for a lifetime of healthy sexuality.