If one defines a graph with each individual representing a node, and an edge connecting two individuals who have had sexual contact, then the large majority are part of a huge connected cluster.

Here is a paper which observes this in a high school. Here is just the graph. An animated gif of the development over time. One thing that disturbs me about the paper is that they make no mention of asymmetric claims by the students. (ETA: actually, they did, see cupholder)