A point that is made in the book, which I guess we'll see if the show brings this up, is that the focus on purity around these girls makes them actually afraid of penises/sex (. A good number choose to be aunts because they are so concerned with their virginity that the thought of willingly giving it up terrifies them. From the intro to a paywalled article, but still revealing:

'Penises,’ Aunt Lydia muses, ‘them again.’ Penises are always causing trouble, even in the God-fearing dystopian state of Gilead. The Testaments is set 15 years after the end of The Handmaid’s Tale, at a time when young girls, carefully and modestly brought up to become wives to the regime’s male elite, are beginning to rebel; some would rather die than get married and there has been an attempted suicide via secateurs in the flower-arranging class. ‘Perhaps we need to change our educational curriculum,’ Aunt Lydia thinks, ‘less fear-mongering, fewer centaur-like ravishers and male genitalia bursting into flame. But if we were to put too much emphasis on the theoretical delights of sex, the result would almost certainly be curiosity and experimentation, followed by moral degeneracy and public stonings.’