COMPETING MALE THEORY. “One likely candidate for influencing the evolution of obligate sex is sexual selection, the process by which individuals of one sex, typically the males, compete over mating opportunities. If individuals pick the best mates they can find rather than mating randomly then sexual populations can gain an advantage over asexual ones.”
http://onlinelibrary.wiley.com/doi/10.1002/ece3.1516/full