In previous experimental studies on laboratory mice, it was shown that the activation of specific immunity by injection with sheep red blood cells (SRBCs) lessens the olfactory sexual attractiveness of male mice to intact female mice. However, reduced attractiveness can only decrease the reproductive efficiency of male mice under the conditions of free mating, which is not obligatory for natural populations. The goal of this work was to study the influence of immunoenhancement on the sexual behavior and reproductive output of outbred ICR male mice. Male mice either injected with saline (control group) or treated with SRBCs were kept with intact female mice for 5 days after injection. While the number of fertile copulations was almost equal in both groups, the potential (ovulated eggs) and actual (number of embryos) fecundity was significantly higher on the 3rd–5th days after injection, when a specific antibody-forming process starts and the scent of male mice becomes less attractive for female mice. Based on the previous and present data, the hypothesis is proposed that, if the quality of a nonalternative mating partner is compromised by the activation of specific immunity, a female tries to maximize its reproductive output (due to the low chance of repeated copulations). This responsibility for the next generation is reminiscent of the Bible story about Lot and his daughters, and may help to sustain specific existence under the conditions of parasitic stress.