The "Heathers," "True Romance," "Untamed Heart" and "Bed of Roses" star said he first met his fiancee in 2009 at Little Palm Island Resort near Miami.
"This girl came walking along with another lady, a slightly older lady," Slater told Fallon earlier this week.
"I was sitting in my usual spot, eating my oatmeal ... and this spot was famous for people proposing to each other. It was very, very nice. So she's there, and the next thing I know, she gets down on one knee and proposes to this older lady," he said. "I thought I had just witnessed a nice lesbian engagement, so I was coming over to say congratulations. But it turns out it was her aunt, and she was just trying to help her aunt capture the beauty and the romance of the moment. She was making a joke, and I witnessed it, and it intrigued me, and the next thing I knew, we were hanging out in the Everglades."
The actor did not say when or where he and Lopez plan to exchange vows.
Slater was previously married to Ryan Haddon, the mother of his children -- Jaden, 13, and Eliana, 11, UsMagazine.com said.