When I delete the [menu.main] identifier from contact-us.md then class="active" doesn’t get added when you are on the Contact Us page.
The [[menu.main]] definition with the weight, url, etc. in the config.toml is nice to have because then I manage pretty much everything in one place (otherwise I am sure I could move the [[menu.main]] entry to the contact-us.md file).
##Edit:
If you have a better way of associating a [[menu.main]] entry in the config.toml with content then please let me know! I might be doing something wrong - I am probably doing something wrong. 
On further review: My original issue only seems to be an issue at all because I don’t follow “convention” on file management. My Contact Us content is located at: content/contact-us.md. The menu system works as expected when you locate your content at the following path: content/contact-us/index.md. I just don’t like having a bunch of folders with a single index.md in each of them.