I understand that a single female being a unicorn is a ls norm but I've always found it kind of funny. The are essentially a horse with a huge phallis on their head. In response to your question, I'd say there are two primary answers. First, let's be honest, a girl doesn't really have to try. She can walk up to almost any couple and ask to join and will most likely get a yes, or at least some moderate consideration. Secondly, most women are raised to believe that promiscuity devalues them and tend to open up and explore more when they have a partner that encourages and supports them to explore their wants and desires.