The argument posed by Penelope Trunk is that startups are overwhelming and emotionally charged stages of a company's life. She makes the distinction between merely growing a "lifestyle business" and undertaking a startup. Then she notes that women are often unwilling to run startups—because they have to put in long hours, and there is a lot of risk involved in the process. They might even cry.
Adding men to the equation, Trunk says, creates more problems. The woman is likely to be more distracted, and—gasp—people might think you and your business partner are a couple. Plus, women might cry in front of the men, and the men might throw a fit. In fact, she goes so far to say, "Most women cry at work. And most guys throw a fit."
In addition, the problems that might be caused by having a man on your team (if you're a man, the problems of having a woman) are compounded by throwing diversity into the mix. Diversity is bad, according to Trunk because, well, there are more ideas in the fire, more ways of getting things done, and more "reality checks." That can seem pretty heavy-handed, but she's arguing that for a startup to be successful there has to be a considerable amount of focus.