They’d have to lie about who they slept with, and I expect the other person might have something to say about that if it was not true.
I wonder if they verified each claim from both sides.
Edit: it’s a scientific paper, so there’s no need to wonder!
In fig. 2, and in all discussions presented here, all romantic and sexual relationship nominations linking students are included, whether or not the nomination from i to j was reciprocated with a nomination from j to i.