ELIZABETH: The very day we announced our engagement, he told me of his experiments. He said he was on the verge of a discovery so terrific that he doubted his own sanity. There was a strange look in his eyes, some mystery. His words carried me right away. Of course I've never doubted him but still I worry. I can't help it.
Henry coincidentally gets caught up in his experiments just as he proposes to Elizabeth. Or is it a coincidence? Maybe Henry has some serious commitment issues. Yes, sure, I'll marry you…but first I have to just run off and make a monster. See ya. It seems a little too convenient.
Remember, too, that director James Whale was gay. Henry's decision to race off to spend time with Fritz could just be bad judgment…but it could also mean that he's not interested in marrying a woman—a fact which most people in 1931 would have kept carefully hidden for fear of prejudice.
VOGEL: What I really want to know is, when will the wedding be, if you please?
BARON FRANKENSTEIN: Unless Henry comes to his senses, there'll be no wedding at all.
VOGEL: But Herr Baron, the village is already prepared.
BARON FRANKENSTEIN: Well, tell them to unprepare.
Henry's wedding isn't just important to his family; it's important to the entire countryside. Marriage in the film is a symbol of social order and harmony. Frankenstein's refusal to marry ends up throwing the whole village out of whack. (Again, you could see this as a kind of fear that homosexuality will cause society to crumble—or perhaps as making fun of that fear.)
BARON FRANKENSTEIN: I understand perfectly well. There's another woman—and you're afraid to tell me. Pretty sort of experiments these must be.
The Baron suspects that Frankenstein is sleeping with someone other than Elizabeth, and that he doesn't want to get married because he is unfaithful. The truth is that he is unfaithful, kind of. He's putting his own ambition ahead of Elizabeth—he's fallen in love with a monster, which is maybe worse than if he'd just taken up with another woman.
FRANKENSTEIN: There can be no wedding while this horrible creation of mine is still alive. I made him with these hands, and with these hands I will destroy him. I must find him. [to Victor] You stay here and look after Elizabeth. I leave her in your care, whatever happens. Do you understand? In your care.
In the original version of the film, Henry dies, and Victor ends up with Elizabeth. So this is the moment where Henry hands Elizabeth over to his rival. You might think, hey, Frankenstein, you jerk: you don't get to decide who she marries. But Frankenstein isn't going to listen to you.
Remember, the whole film is about him trying to create life by resurrecting dead bodies, rather than by just getting married and having a kid. He wants to control stuff, especially stuff having to do with women, like birth and marriage. Really, it's hard to know what Elizabeth sees in him.
BARON FRANKENSTEIN: Well, as I said before, here I say again, here's... Here's to a son... to the House of Frankenstein.
MAID: Indeed, Sir. You too, Sir.
This final line of the film lets you know all is well with the world. Henry's getting married, he'll have kids, and he'll make them the normal, natural way, without the electricity and dead bodies. Marriage restores order, and even the maids are happy. (Though, for a less cheerful take, look at "What's Up With the Ending?")