"La, madam," cries Honour, "you will make a very bad examiner. Hark'ee, child," says she, "is not that very young gentleman now in bed with some nasty trull or other?" Here Susan smiled, and was silent. "Answer the question, child," says Sophia, "and here's a guinea for you."—"A guinea! madam," cries Susan; "la, what's a guinea? If my mistress should know it I shall certainly lose my place that very instant." "Here's another for you," says Sophia, "and I promise you faithfully your mistress shall never know it." Susan, after a very short hesitation, took the money, and told the whole story, concluding with saying, "If you have any great curiosity, madam, I can steal softly into his room, and see whether he be in his own bed or no." She accordingly did this by Sophia's desire, and returned with an answer in the negative. (10.5.6)
The main problem with Tom's sleeping around is that he almost ruins his relationship with Sophia, the woman he claims he loves. So it seems like Fielding is arguing that the difficulty with totally giving in to your physical desires isn't what it does to "society" or abstract ideas like that. The issue is that, by being unfaithful, you can hurt the person you love (especially if that person expects you not to sleep around). And the one who ultimately almost pays for his lustfulness is Tom himself. After all, if Susan hadn't spotted him with Mrs. Waters at the inn at Upton, his issues with Sophia might have been resolved right then and there.