You can hear Gavin and Stacey having sex. It is only the sound of a bed rocking and it is mainly played for comedic purposes.
In season three, episode three, a man is planning a party for his nephew in laws friends and he puts a newspaper on the wall with a woman with her upper body nude. He says "It's for them to stare at." A woman is quick shocked by this.
Strong references to threesomes in series 2.
We hear that a main character went on a fishing trip with his nephew and something happened that is not revealed but by what is spoken about it it may suggest sexual activity.
Two sex scenes. One where a man is thrusting behind a woman (fully clothed) by a table for around 30 seconds until the woman stops him and another where a man is also thrusting a woman on a chair for 10 seconds until they're interrupted.
Gavin is sitting on his bed and he can hear a man and a woman having sex in another room for just around 25 seconds but then it all stops.
The second and third series is filled with mild to moderate references about pregnancy.