Only gradually does Tom realise how morally corrupt Lady B. is - he didn't know of her inciting Lord Fellamar to rape Sophia. Lady B. is cynical about marriage - not believing in love, she only sees property marriage, and wants to protect herself against it. 622-623 is important for distinguishing between Tom and the code of the male rake, equivalent to Lady Bellaston's. His repentance is over-determined, because he is also getting someone he desires sexually, and who is rich. To put it cynically, Fielding makes it convenient for him to be virtuous from now on.
Some women are without morality, but only a woman can be independently moral.
Allworthy's call for a single standard of sexual morality therefore remains impractical, and Fielding's conclusion may be condemned as sentimental, and as perpetuating stereotypes about male and female attitudes. But perhaps we should just sit back and enjoy the kind of ending that fairy-tales promise.