Sex, love, and commitment.
When a couple become erotically pair-bonded, whether for a night or a lifetime, they are by definition committed to each other erotically. That is the minimum. The degree varies. The commitment may be limited to the proceptive phase of solicitation or courtship, or it may include the acceptive phase of coitus, or it may extend to the conceptive phase of parenthood. People "fall in love with their fantasy" and project onto the partner a range of future commitments. If the partner fails to reciprocate, the pair-bond weakens and the partnership either dissolves or becomes eligible for marital counseling or sex therapy.
D000818:Animals; D001143:Arousal; D003380:Courtship; D004285:Dogs; D005260:Female; D000882:Haplorhini; D015186:Hominidae; D006801:Humans; D007398:Interpersonal Relations; D008149:Love; D008297:Male; D012723:Sex