If you were expecting an it's-finally-happening-they're-making-out-so-much-eroticism explosion between Frederick and the now-widowed Marie Arnoux, prepare to be sorely disappointed.
Yes, the two romantic leads are reunited, but nothing sexy goes down. Frederick clearly still has deep feelings for the lady, but he just can't see himself sleeping with her. The fact that she has a head full of white hair doesn't help, but it's not really about that.
Let's take a look at his struggle:
Frederick had a suspicion that Madame Arnoux had come to offer herself to him, and once more he was seized with a desire to possess her—stronger, fiercer, more desperate than he had ever experienced before. And yet he felt, the next moment, an unaccountable repugnance to the thought of such a thing, and, as it were, a dread of incurring the guilt of incest. Another fear, too, had a different effect on him—lest disgust might afterwards take possession of him. Besides, how embarrassing it would be!—and, abandoning the idea, partly through prudence, and partly through a resolve not to degrade his ideal, he turned on his heel and proceeded to roll a cigarette between his fingers. (2.19.26)
Wow. So after all that pining, he's "suspicious" that she's come to do the one thing he has wanted and been denied for twenty years. What? Why "suspicious"? Well, it seems like he feels too close to her—in an emotional and spiritual way—to become sexually involved. It's almost like he doesn't want to break the spell by getting to know her as anyone other than Madame Arnoux, the unattainable woman. Oh, and he's worried that the sex wouldn't be good, which would totally ruin his ideal vision of his crush.
And that, Shmoopers, is Frederick being sentimental. Check out "What's Up With the Title" for more on that excitement.
But Wait! There's More
After Madame Arnoux makes like a tree, Flaubert turns briefly to one final exchange: between Frederick and Deslauriers, his oldest friend.
The two men discuss their youth, particularly one time when they'd been seen trying to sneak into a brothel (Though, as it turns out, they'd been too afraid (again!) to sleep with a woman anyway. Frederick's final words are: "That was the best time we ever had!"
Welp. That's depressing. And kind of anti-climactic, don't you think?
It's kind of indicative of their lives, though. Ultimately, neither of these men had much success in experiencing life directly. And it looks like neither of them have changed much, either.