She married soon thereafter, the selfsame man who accused her of sublimity, and gave him merry hell for many years. (1.11)
As Goldman shows us, marriage isn't always the happiest of things. In fact, there are more characters in this book that regret getting married than ones who don't.
Buttercup's parents did not have exactly what you might call a happy marriage. All they ever dreamed about was leaving each other. (1.38)
Once again, Goldman shows us two characters that can't stand being married to one another; there are also several other points in this book where Goldman talks about his own marriage as being cold and unemotional. So yeah, there's not a whole lot of love for the institution of marriage.
Her tone was surprisingly tender, and probably she sensed how important he really was to her, because when he did die, two years further on, she went right after, and most of the people who knew her well agreed it was the sudden lack of opposition that undid her. (1.53)
Even though Buttercup's parents are always at each other's throats, it's actually their fighting that keeps the two of them going. Once one of them dies, the other one doesn't really know what to do and dies soon after. It's kind of a touching detail in its own way.
"Drat! […] That means I shall have to get married." (2.22)
Prince Humperdinck isn't a fan of marriage—he'd much rather spend all his time hunting and working on his hobbies—but once he finds out his father is dying, he realizes that he'll have to get married so he can have a son and provide the kingdom with a new heir.
To Buttercup's way of thinking, there were two main problems: (1) was it wrong to marry without like, and (2) if it was, was it too late to do anything about it. (5.29)
Buttercup loves Westley with all her heart. But now that she thinks he's dead, she looks at marriage in a very cold and calculated way; she even starts listing pros and cons for marrying a man she doesn't care about at all. Oh well, she's already agreed and it seems that there's not much she can do about it now.
I spent that whole night thinking Buttercup married Humperdinck. It just rocked me. (6.100)
William Goldman explains that when he was a kid and his father was reading The Princess Bride, he nearly lost his mind at the thought that Buttercup had married Humperdinck. But this is just one of the book's many fake-outs, because this scene is actually one of Buttercup's dreams. That said, Goldman is quick to remind us that not everything is fair in life, and good guys don't always win.
"Widows happen. Every day—don't they, Your Highness?" (8.75)
Turns out that Buttercup actually does marry Humperdinck. But Westley isn't too concerned, because he's pretty confident that Buttercup will soon be single again (once he kills Humperdinck).
"I think, sweetest child, that we should strike a bargain, you and I: if Westley wants to marry you still, bless you both. If, for reasons unpleasant to mention, his pride will not let him, then you will marry me, as planned, and be the Queen of Florin." (6.137)
Humperdinck is a liar, plain and simple. He knows full well that Westley is shut up in his dungeon, but he's still willing to pretend that Buttercup still has a chance of marrying the guy. But hey, that's just the kind of guy Humperdinck is.
"He couldn't be married. I'm sure. Not my Westley." (6.138)
When Humperdinck suggests that Westley might have already gotten married during his years away from Florin, Buttercup doesn't want to believe him. Then again, people change over time, and she feels compelled to write a letter to Westley just to double check whether he still plans on marrying her.
"I'm not a witch, I'm your wife […] and after what you've just done I don't think I want to be that anymore." (7.194)
Miracle Max's wife Valerie isn't a woman who'll just sit back and let her husband do whatever he wants—when she thinks he's being a jerk, she's quick to jump up and tell him. Luckily for us, she tells him to bring Westley back from the dead.