As Franklin describes himself, he's frugal and full of virtue, so there's not a lot of steaminess in this book. While he makes a few asides about spending time with prostitutes – he says he's lucky that he didn't catch any STDs – Franklin really stays away from talking about sex for the most part. He doesn't even talk about his wife that much, and when he does mention her, he just says she's a "good and faithful helpmate" (1.108) who is like a partner in his work. As a married couple, they're committed to trying to make each other content, but Franklin doesn't dwell on any physical parts of their relationship.
In his list of virtues, he puts Chastity as number 12, and says of it that you should "Rarely use Venery but for Health or Offspring; Never to Dullness, Weakness, or the Injury of your own or another's Peace or Reputation" (1.42). In other words, Franklin says here that sex is for procreation and should be used in moderation, even if that's not something he follows throughout his private life.
Finally, in a move that points out the difference between his autobiography as a work of literature and his actual life story, he doesn't discuss the other relationships he had with women or how that lead to children outside of his marriage.