There's a lot more going on here than mostly cohabiting, paying mortgages, and collecting Airbnb rent, isn't there? "Recently found out?" "Feel taken advantage of?" A feeling like the goose and the gander aren't getting the same? Either you're not as close a couple as you might imply, or (as is usually the case in these things) there's a whole lot of communication that isn't happening.

But be that as it may, you both own your own places, and, surprise, one of the perks of ownership is, to a large degree, being able to make your own decisions about how to run the place. She wants to Airbnb hers? That's her thing (unless the city steps in with its hoteliers' codes, but that's a different issue), and she's got no obligation to share the profits, at least from what you've told us. If you don't feel free to do the same, then that's an issue.

But I would caution against taking Abby's advice here, unless you know you want to steer your relationship into the cliffs. Opening the currently limited lines of communication with this topic is a pretty good recipe for having them snap back closed, hopefully not with your head caught in them. Instead, try to strengthen your communication skills in other ways - you know, basic things so that you don't have to "recently find out" about potentially important things, but that they're part and parcel of the stuff you talk about every day. Because I see no relationship here, just two people sharing space.