The pages in the main menu are CMS pages and they use url re-writing to make friendly urls like /home.aspx or just /home if using extensionless urls. These friendly urls are all re-written to /Default.aspx?pageid=x (which is the real page that serves the request) but the user only sees the friendly url in the browser.
However since Default.aspx is the real page that serves content for all theses friendly urls you cannot change it.
For other physical .aspx files you "might" be able to create friendly urls that map to them in the Administration > Advanced Tools > Url Manager.
For example you could try making a friendly url ~/sitemap that uses the real url /SiteMap.aspx
However the built in links are not going to know about those friendly urls so they are still going to render the real url. So it would be impractical because you would have to create your own links that use the friendly urls you define. to replace the links for sitemap and other things to try and hide the .aspx extensions.
So, the bottom line is, if you have a customer that insists no urls can use the .aspx extension then mojoPortal is probably not the solution for that customer.
If you were hand crafting html pages instead of using a CMS they would have a .html extension, so it is not much different to have a .aspx extension.
Maybe at some point in the future I will try to use ASP.NET routing to alllow extensionless urls on these physical pages, but, there are still people using IIS 6 which cannot use extensionless urls without third party isapi modules. IIS 7 is still relatively new, and the possibility for extensionless urls only became possible easily in IIS 7.
For now, you can only use extensionless urls for CMS pages easily.
Best,
Joe