Oh dear. This episode is an embarrassment for the Whoniverse. The first ten/fifteen minutes are great, as you see something zooming across the sky and Gwen accidentally release alien gas, but soon as it's revealed to be a sex alien it just becomes laughable.

It's basically just a fifty minute excuse to shout 'Hey, we're an adult show! We can do adult things that Doctor Who can't!', when in actuality how adult a show appears comes from how mature it is rather than because it explores adult topics.