My wife's practiced reply for this is: "I only pedal if he steers!"

It's kind of a dumb comment, and at least to my thinking, it leaves the field open for all manner of responses on a similar level.
As capt I sometimes come back with stuff like ...
I'm the brains, she's the brawn.
Hey I'm only here for my good looks, she's doing all the hard work.
etc

Tandems are still an eyecatcher + a rarity, and because of that, they still provoke spontaneous comments that just come out without thought. So few of the people who are NOT on tandems have the FIRST understanding of what's it's like to be on one.

*** We are the lucky ones. Most others just dunno what they're missing or what we're enjoying.