This article develops the Affective Ship Hypothesis, which suggests that women experience positive affective shifts following first-time intercourse as a means to facilitate a longer-term, more committed relationship. The hypothesis predicts a negative affective shift in men who pursue a short-term mating strategy; this shift is hypothesized to function to curtail commitment by motivating the man to terminate the relationship. Study 1 (N= 177) documented sex differences predicted by the affective shift hypothesis. Study 2 (N= 203), using a somewhat different methodology involving reports of presex and postsex feelings, found that men with high numbers of sex partners, but not men with low numbers of partners, experienced a decrease in their partner's physical and sexual attractiveness following first-time sexual intercourse. In contrast, women, more than men, experienced increases in feelings of love and commitment following first-time sex.