Let's see, in the first shot Fry is wearing pants, Amy is standing a good distance away, Zoidberg and Farnsworth is there, nothing speacial.
In the second shot Fry is suddenly in his boxers, smiling akwardly at Amy, who doesn't look all to happy with him. Zoidberg and Farnsworth has gone.
If you didn't watch the show, you might get the idea Fry made a pass at Amy. And who could blame him (except the shippers that is
There are a few shots without Fry in the sequence. Zoidberg probably removed the pants to show him the missing dorsal fin.