"At sixteen, Madame de Rênal, rich heiress of a devoted aunt, was married to a well-bred gentleman, having neither experienced nor witnessed, at any time in her life, anything that resembled in the slightest the word of love." (1.7.59)
Madame de Rênal has no clue what love is. She got married very young and never really thought about it all that much. This is all just a setup for the moment when she realizes that she's in love with Julien Sorel.
Suddenly, a word frightened her: adulteress. She could see it. The worst things that the vilest debauchery could stamp on the notion of sensual love swarmed into her mind. (1.11.14)
Madame de Rênal has spent her whole life doing everything right. That's why she's so disgusted and terrified when she thinks of herself as an adulteress. Then again, she knows that if she had it all to do over again, she'd still be with Julien.
"I'm not going to deprive myself of my wife, she's much too useful to me." (1.21.22)
Monsieur de Rênal ain't exactly the romantic type. Even when he's presented with the thought that his wife is having an affair, he can't bear to get rid of her. But this isn't because he loves her so much, but because he finds her "useful" and easy to get along with.
All the same, there had been deadly days when she could not keep from picturing the enormous happiness she'd taste if, having become a widow, she was able to marry Julien. (1.23.66)
Madame de Rênal is a moral woman. She knows she'll never be able to live openly as Julien Sorel's lover. But she does fantasize about her husband dying so that she can respectably be with Julien. She doesn't seem to realize that it's not all that nice to fantasize about people dying.
What strange effects marriage has, in the form practiced by the nineteenth century! The boredom of married life certainly kills love, if love has preceded marriage. (1.23.68)
Stendhal isn't giving us a ringing endorsement of marriage here. He directly tells us that getting married tends to kill whatever love two people used to feel for each other.
"And I'm going to seduce his daughter—me! Perhaps her marriage to de Croisenois will be impossible—a marriage that glows in Monsieur de La Mole's future." (2.13.24)
Julien Sorel feels his ego glowing when he thinks about seducing Mathilde de La Mole. He gets a real kick out of the thought that sleeping with her will ruin her chances to marry a respectable man like her father has planned. Julien usually feels grateful toward the Marquis de La Mole, but here we can see his hatred of the upper classes shining through.
"Really, he's my husband […] If I make a good-faith return to believing in wisdom and honor, obviously it's him I ought to marry." (2.25.38)
Once Mathilde has had sex with Julien Sorel, she believes that she is his wife in the eyes of God. So they might as well make it official with a real wedding. Mathilde knows that her father is going to disapprove, but she's willing to risk his anger in order to make good on her commitments.
"Your honor is protected: I'm your husband. This truly major step will change everything, for both of us. I too am within my rights." (2.32.25)
It doesn't take much convincing for Julien to agree to marry Mathilde. He knows that the marriage will give him all the status and power he could ever dream of. Oh yeah, and he's pretty fond of Mathilde, too.
"It's only true on the surface […] She's my wife, but she's not my beloved…" (2.43.21)
When he meets with Madame de Rênal following his conviction, Julien tells her that in his heart, she is his one true beloved. He admits that he has agreed to marry Mathilde, but insists that she's not the one he loves.
"My primary responsibility is to you […] I've escaped from Verrières." (2.45.3)
Madame de Rênal is willing to risk everything just to spend a little more time with Julien before he dies. She even runs away from home and gets all of Verrières gossiping about her. She doesn't care, though, because she's madly in love with Julien.