The vagina is a necessary part of procreation, and thus a necessary organ in regards to sexual activity. The breasts are a specific feminine feature which specifically trigger male reproductive hormones.

The anus is not. It is a forbidden asset, hidden from the light, and properly used only for the excrement of waste. It is meant to by hidden from, and is seen only when at the individual's most vulnerable position. That is why the anus is the most beautiful part of the female body.