Molly is Tom's first love. She is the daughter of Black George the gamekeeper, and famous in the area for her attractiveness. But the narrator notes that there is something about her that "would at least have become a man as well as a woman" (4.6.15). Molly is "bold and forward" (4.6.16), rather than modest and shy.
In other words, Molly is the absolute opposite of Sophia Western: where Sophia is delicate and feminine, Molly is rough and even masculine. Where Sophia is chaste and careful in her interactions with men, Molly has at least three sexual partners that we know of: Tom, Mr. Square, and Will Barnes. And of course, the biggest difference is that Molly gets pregnant outside of marriage, while Sophia has to spend much of the novel caught up as a pawn in a competition between Mr. Blifil, Lord Fellamar, and even Tom for her hand in marriage.
What we find interesting about Molly vs. Sophia is the obvious difference that class makes in the way Fielding talks about these two women. Molly is relatively open about her sexuality, and she clearly enjoys her body—like, for example, when Tom sees her returning from her work in the fields looking all sweaty, and the two quickly decide to have sex in a nearby clearing. But her sexiness is partly because she is a poor woman, and so the social rules that govern her life are a little different from Sophia's.
While Molly's mother is certainly upset that Molly is having a child outside of marriage, her worries seem to be more financial than moral. When Molly shows her mother some money she received from Tom, everything seems to be all right between them. And at the end of the novel, Sophia is arranging a marriage between Partridge and Molly. So Molly's illegitimate child does not stop her from being a possible match—this definitely isn't The Scarlet Letter.
By contrast, Sophia is not just a woman in her own right. She is also Squire Western's heir. It's her money that makes her so attractive to Mr. Blifil in the first place. Lord Fellamar is also pleased to hear that she is going to inherit a fortune. Sophia's money is also what makes Tom an impossible candidate to start with: while Molly Seagrim bounces back from having an illegitimate child, Sophia is in a high enough class position that she cannot marry an illegitimate son.
We mentioned "Gender" as one of our ten major themes in this novel. But it's worth noting that class has a huge influence on how women are presented in this book. The novel's representations of poor Molly Seagrim and rich Sophia Western are so wildly different that the fact that they share a little thing like gender almost doesn't seem to mater.