Pure cowardice
IMO, it was pure cowardice on the part of the movie that the woman did not have sex with both men, (at different times).
Two career spies? Dating the same woman? It would have been easy to manufacture a scenario where she sleeps with one and then the other thinking that they are unavailable for long term relationship because of "reasons",
and then the situation changes for both of them, putting her in the awkward situation.
Especially as htey are both agents working together. They would both make the transition from world traveler, to stationed at HQ, at teh same time.