I kept looking at that too. It's crazy to me that there are only 2 cycles in the graph and one is the big accidental one. It honestly makes me think that either something must be wrong with the data, or it's reflective of some deep principle of math or sexuality (e.g. that people won't fuck around within their close social grouping nearly as readily as they will with people on the outskirts of it).
The authors wrote that they were surprised too and went back to talk to the students and apparently there was an unwritten rule that you don’t date the ex of the new partner of your ex. So if Bob and Alice split up and Alice starts dating Ben, then Bob should not date Ben’s ex Alison.