Even without pairing off, simple age dynamics skews it in favour of women first and then men. Young women statistically want to date older men (even if only by a few years) since older men are more capable, confident, and have far more resources. Men want to date younger women (usually again by only a few years).The end result is that men have a much tougher time competing up until mid-20s, and then roles suddenly reverse and women in their late 20s and beyond are left wondering 'where all the men went'.