I agree that the initial involvement would be very harmless. The problem is that by putting 13 year olds who are raging through puberty together without the wisdom that age brings invites the potential for poor choices. Because 13 year olds do not always think things through.
Having sexual feelings is a normal human experience. But it is better to wait until older before engaging in a one on one dating experience that may very well place two young persons in a situation where they will not be able to stop.
The only rule that is being enforced is the one on one dating rule. My son can certainly hang out with the girl of his interest .. just not by himself with her on a date. He can hang out with her while bowling with a group of friends - they can be together without the burden of behaving like "boyfriend"/"girlfriend" and all that may imply.
I really don't think putting 13 year olds in a dating situation is part of normal adolescent development. I think that normal adolescent development means that the hormones are raging and the interest is there .. but there is no wisdom and no restraint there yet - because of the youth.
I just have struggled with how to communicate to other parents about how we feel about the young dating situation. One of the moms just called and wants to setup a group trip to one of the local highschool musical productions .. I of course volunteered to either pickup or drop off ! So maybe other parents are going to follow along - we shall see.