In Ernest Hemingway's A Farewell to Arms, the only area I can imagine where one might logically look for a "biological trap" is in the relationship between Catherine and Frederic. Literally, this might refer to Catherine's pregnancy, however, it might also relate to the dependency that Frederic develops for Catherine when he is wounded.
If there is a "trap," one might point to Catherine's transfer to the hospital where Frederic is taken when he is wounded in battle. Catherine and Frederic have already starting seeing each other. When he finds himself in the hospital, it might be easy to assume that Catherine is trying to ingratiate herself with Frederic when he is at his most vulnerable.
Once Frederic decides to desert the military as he sees men being killed by their own officers, he takes a pregnant Catherine with him to Switzerland, where they will be distanced from the war and safe from the arm of Frederic's superiors because he is now a deserter. The two sweethearts spend a great deal of time together. Catherine's pregnancy advances, and when she goes into labor, Frederic takes her to the hospital. There the baby is born dead, and soon after Catherine bleeds to death.
While some readers made find Catherine's assertiveness suspect, as if she has ulterior motives, it would seem that if she was simply looking for someone to take the place of her childhood sweetheart killed in the war, she could easily have formed a relationship with Rinaldi. However, we can also assume that while we see Catherine through Frederic's eyes, it seems that both Catherine and Frederic are interested in developing a relationship with each other.
Catherine is seen as a woman typical of those who loved men involved in the war.
Catherine is a close examination of femininity in wartime...Catherine is a representation of women in war, both as the ideal being defended by the army and the ideal sought by the individual soldier.
Catherine's assertive behavior with Frederic does not indicate a trap of any kind (biological or otherwise). She is simply willing to pursue Frederic when he is slow to move. And as this exceptional woman dies, she is noble and unafraid, giving us a glimpse into the real woman beneath Frederic's perceptions of her. Tragically, Frederic is left alone, heartbroken, to face the future.