The evolution of sex is a tough riddle. But even harder to explain is the evolution of obligatory sex: how come that for so many organisms sex is the only mode of reproduction? I will present two works related to this question. The first demonstrates the severity of the problem, by considering organisms that can switch to sexual reproduction according to their condition - reproducing sexually when their condition is poor, and asexually when their condition is good. We show that the advantage of such facultative sexuals over either asexuals or obligatory sexuals is great. In the second work, we show that sexual selection can help the obligate sexuals, and that under strong sexual selection obligatory sex can evolve even in direct competition with facultative sex. Together, these works suggest a two-phase evolutionary path from asexuality to obligatory sex.