The US security services are purple with shame this week. Not only have they had to endure a prostitution scandal, now everywhere they turn they're faced with an audacious ad campaign from Spirit Airlines that won't let them forget about it.

The low cost airline's "More bang for your buck" campaign features a secret agent in front of four bikini-clad women. The not-so-subtle reference is to the disclosure that US military personnel and Secret Service agents were caught in hotel rooms with at least 20 women in Cartagena, Colombia, shortly before President Obama arrived in the city for the Summit of the Americas. Eleven agents have been put on "administrative leave".

The affair came to light after a loud quarrel between a sex worker and an agent, after the man allegedly refused to pay her in full. Perhaps that's why the airline ad specifies that payment is required upfront?

Take action: Spirit Airlines may be making a joke at the expense of the Secret Service, but for thousands of vulnerable women and girls across the globe (Colombia included), prostitution is no laughing matter. Read more stories here about how thousands of vulnerable women are trafficked and sold every year.