The fastest way I can think of to do this would be to create an internal URL rewrite of your homepage in your .htaccess file.
1. Find your .htaccess file. It should live in your Magento installation directory.
2. Find the section that says
3. Somewhere below that, try the following:
# Internally rewrite homepage URL to real file location
rewriteRule ^$ /categoryname.html [L]
Where /categoryname.html is the url of your men’s shirts category.
This isn’t a rewrite 301 redirect, but instead an internal rewrite. Also, an external redirect and an internal rewrite are different things; An external redirect involves the client (browser) and the URL change is visible to the user, whereas an internal rewrite changes only the filepath associated with a URL, and is not visible to the user. This should be fine for SEO.
I haven’t tested this in parallel with Magento’s config, but hopefully this (or something similar) will work for you…