> when is it harmful to see people participating in sex acts?

I'm no conservative, but kids copy what they see, even before they understand language and responsibility. There are no absolutes, but it's wise to moderate showing sex acts to children until it's likely they won't repeat it responsibly.

Without commenting whether it is harmful or not, showing pornography to children is considered sexual abuse by law in many countries.

Two girls, one cup. That hurts the stomach muscles...

Other than that it isn't :)