If you're a male who decides to stay abstinent until either marriage or a committed relationship, the exception made to only engage in cunnilingus (oral stimulation on the female genitalia) as a form of erotic foreplay or as an alternative to sex.
This may seem highly hypocritical to many people since "oral sex" is viewed as being just as intimate as vaginal intercourse, but the idea is keeping your own private area "pure" until you have either met the right person and/or have gotten married.
The cunnilingus exception
is not for everyone, but it is a personal lifestyle choice that a few people (including myself) have made. This can also be for religious, spiritual, or philosophical reasons.
The cunnilingus exception is like "having your cake and eating it too". Pun intended. :-)