That is only part of the stanza. The complete paragraph states that there will be a total of 100 virgins: 72 FEMALES and 28 BOYS.
Freepers, in their generosity and tolerance, oftentimes pass up that fact, in deference to one of the great religions of the world.
I know because I made the same mistake myself.
It is a common assumption among many imams that the stanza about being able to orgasm 100 times a day, and the stanza about there being 72 female perpetual virgin spirits, are reconciled by the “young boys scattered about like pearls” being called upon to make up the extra 28 to fill the ‘100 times a day’ quota without having to do any of the female spirits twice in the same day.
Nowhere is the actual number of “28 boys” stated directly; it is the difference between the “100 times a day” and the “72 perpetual virgins”.
At least when challenged to support my assertion (identical to your own) I was unable to find any direct source.
Let me know if you find one, otherwise it is a common interpretation, but not directly stated.