They are posing like female pin ups pose. It is supposed to make people think about how men and women are represented differently in images like these.
Let me try another way to say it.
When men pose in this way, it is silly, funny, lighthearted, whatever. When women do it, it's not silly, funny, lighthearted, its normal because women tend to be presented in this way for men's sexual titillation.
You might not understand this, doesn't make it untrue.
I am bemused by the growth of cheerleading in this country. Our nanny was in the cheerleading team at her university (a very good one, though not oxford or Cambridge). no such thing existed when I was an undergraduate. if anyone had asked me or any of my female friends if we wanted to be cheerleaders we'd have laughed ourselves sick.