The immediate cause is Major Petkoff's request that Catherine accompany him to talk to the men in Act III. Bluntschli had just told Sergius to take a strict, forceful tone with his soldiers:
Tell them that if they stop to drink or tell stories--if they're five minutes late, they'll have the skin taken off their backs.
Then, doubting Sergius's ability to follow through, Bluntschli urges the Major to go with him:
Just see that he talks to them properly, Major, will you?
Petkoff first makes a show of being ready to do so, but then seems to lose confidence and asks his wife to help with the task:
Quite right, Bluntschli, quite right. I'll see to it. [He goes to the door importantly, but hesitates on the threshold]. By the bye, Catherine, you may as well come too. They'll be far more frightened of you than of me.
And that sparks Bluntschli's comment. But we should keep the background in mind. Bluntschli is already well aware that Catherine exerts dominance over her husband in the domestic sphere. Bluntschli took cues from Catherine regarding the need to deceive Petkoff and Sergius about his prior visit to the house. He has observed Catherine directing Petkoff in the library:
CATHERINE [in a low warning tone] You can stop interrupting, Paul.
Bluntschli is also well aware that Petkoff isn't very competent at his job. Bluntschli had negotiated with him at the end of the war, an encounter in which Bluntschli "humbugged" Petkoff into "giving him fifty able bodied men for two hundred worn out chargers." And now, in the library, he was helping Petkoff sort out the logistics of sending the cavalry regiments to Philippopolis -- a problem that seems to be entirely beyond Petkoff's capabilities.
So when Bluntschli makes his comment about wives, it comes in the context of his being dismayed with Petkoff's apparent inability to handle anything on his own. More generally, this reaction is part of his dismay towards the unprofessional approach taken by Bulgarian military officers.