Im confused on gavin and stacey episode?

well on series 3 episode 2 every one goes round gavin's house for a curry they all get drunk and then in the morning gavin gets a text and he says oh my god and shows stacey then stacey is like oh my god so she gets her phone and sends it to vanessa then vanessa gets it and she has a shocked face and as she goes to show smithy he already has it and he looks upset im confused why are they texting in the first place and what are they shocked about