YAY! This solved my problem. Or, really, I should say that the reverse of this solved my problem.
Aperture➞Preferences➞General➞Enable Gestures" was indeed checked. I unchecked it and then set up Aperture specific 2 finger gestures in the amazing app Better Touch Tool (I set 2 finger swipes to trigger forward and back arrow keys).
Problem solved! HOORAY!!! Thank! You! Kirby!
I think Apple has set up 2 finger swipes for Aperture to be some sort of forward/back for photos you've viewed rather than forward/back through thr album you're in. For me, sometimes it worked but sometimes it didn't, and figuring out why was driving me nuts. Now, it's set the way I want it and it works every time.