To me it looks like Kateřina is taking František to herself so I tought it would be František getting married to Kateřina but it is the other way around. Really? What a tough one.
Sometimes our logic fails us! Here, though, we have two clues: (1) Kateřina is the nominative case form, so she is the subject, and (2) Františka is in the accusative in this sentence, so he is the object. You can think of it as "Kateřina is marrying František."
I do read it like Kateřina is marrying František, but in this sentence Kateřina is the active person while František isn't but in "Kateřina is getting married to František" it looks to me as Kateřina plays a more passive role