I'm thinking that a possible solution lies somewhere between the two good posts above.
It does sound like it could be normal curiosity - but the question would be from where has it come from. Does he have any brothers or sisters? This might rule out what Mark is suggesting.
In terms of stopping the behavior. You attempted to change his behavior quite a while after it happened. With children of this age - that usually doesn't work very well. The consequence needs to be immediate, (usually short), and consistent. And even then it probably will take a few sessions of timeouts before it clicks. Thus, you probably should not have really expected it to have worked the first time.
The baby sitter needs to know that if this happens again. He should get an immediate time out and maybe not be allowed to play with the little girl for the rest of the day. In other words, she does need to be doing the behavioral modification. Of course, she should also let you know what is going on. I would think that if she does not get to crazy about his behavior. Instantly deals with it in a calm manner. That it would stop after a few attempts.
Let us know what happens. Best wishes!
My concern is that this behavior occurred at least twice, the second time after a significant discipline. I don't think it is sexual due to his age but I do think it is a little more than simple curiosity. I think that it would be appropriate to get this behavior evaluated by a mental health professional. This behavior could be a sign that your son was or is being abused.
Because of the age of your son, I will assume curiosity, not sexuality. Those on the forum who have children of both sexes will probably have had them both naked at the same time. I used to bathe my two eldest, who were just 14 months apart, together. Each always knew what the other looked like and showed little interest. But when the elder, my daughter, was an infant and I had to go back to work, I left her with my sister who lived nearby and who had two young sons. When she was changing my baby's diaper, the boys couldn't understand why she had missing bits. My sister explained that girls were made differently and that satisfied them that nothing was wrong with her.
I am sorry that your little boy has never seen a naked girl. It is only natural that he should wonder what she looks like. But when you punish him for wanting to know, you are telling him that naked is bad, and even asking about being naked is bad. And maybe something about girls is bad. You may not realize it, but you are equating his interest with sexual perversity. Think carefully about how you are handling this. There are some seeds you don't wish to sew.