I respectfully disagree with your interpretation of the study. I concede that this study is indeed strange – I think they may have have been trying to evaluate theoretical desirability and not the willingness of women to actively poach.

Desirability of men rises when they have the following attributes (in my opinion):
– willingness to commit
– ability to feed a family (old-fashioned attitude, yes, but very common thought)
– ability to maintain a relationship

All of these qualities are ‘signaled’ by a man being in an existing relationship which implies that these qualities were tested by another woman.

Therefore, in my humble opinion reason 1 you cite is more likely to be the correct interpretation of the study.

Yours,
Bettina