Mostly its ok, but sometimes they do have some really sexual episodes. Marge and Homer trying to get their "thang" back is one that always sticks on my mind. But there have been several episodes with bits in the have involved H&M getting down to it, or talking about doing it, or being interrupted just before they do it.
Its a cartoon and kids love cartoons and as it is on Channel 4 and Sky at teatime, you could be forgiven for thinking it would be ok for them to watch it. But it does make me uncomfortable sometimes, I have noticed that they used to say "nsuggle" but now they just say "sex" which is not something I want my little ones watching and asking about just yet! After a particularly inappropriate part of an episode I have finally banned it, much to their disgust! DH thinks I am over reacting saying its "just a cartoon" but he wouldnt let them watch South Park and that is a cartoon too!
AIBU and a prude?