I hadn't noticed that before either, but I do see it on the main page: https://www.marriott.com/default.mi

if I scroll down past the "Recently Viewed", and after clicking in to the Destination field to expose the 2nd row of inputs.

I think I normally click on "Find and Reserve" in the top banner though, which brings up a similar but different form which (at least right now) lacks the "I'm traveling for leisure" checkbox, and could be why I don't remember seeing it before...