Besides all the issues mentioned, such as dolls, music shows etc I think sexual messages are actually happening right from birth. I have posted about this before and no offense to others who purchase this clothing but I find alot of the slogan's on baby jumpsuits and todder t-shirts appauling - 'i'm a tits man', 'the condom broke', 'blondes have more fun', 'caution - blonde thinking', 'i-pood'.
These are sexual/adult messages on our babies, children see this and realise that an adult has dressed their baby in this type of clothing and the message is reinforced.
It is up to us to role model what's appropriate for our children.
I also think that some children's bikini's are very sexual but I also think that sometimes it's ok to wear a 2 piece when it's really hot (as it gets here) and you are just running under the hose in the shade. I played in my undies in the yard and it certainly wasn't viewed as sexual. Teaching your child to have self respect and a healthy body image doesn't mean they must always be covered from top to toe.