Lesko and the Girl had sex/mated

...that's what the ants wanted them to do, and if you watch the original cut ending (but also if you pay close attention you can get it from the original release of the film), you can see this. They embrace, and at one point suddenly Lesko shirt is off and he's in a position like he's on top of her. In the cut ending this is more clear, and at one point she is naked, and the images suggests them "melding together" which is just visual metaphor for them having sex/mating. The ants wanted them to #@$% and reproduce a new species of human, that would live like ants. She was the new "Queen Human/Ant", and they you see this new Human Ant world.