Loading two external files into my routes.rb causes issues in Rails 4 with instance_eval

I separate my routes into two files ( marketing_site.rb and subdomains.rb ) which I load in a way that Rails 4 objects :

class ActionDispatch::Routing::Mapper
  def draw routes_name
    instance_eval(File.read(Rails.root.join('config', 'routes', "#{routes_name}.rb")))
  end
end

MyApp::Application.routes.draw do
  constraints(lambda { |request| Config.main_site? request }) do
    draw :marketing_site
  end
  constraints(lambda { |request| ! Config.main_site? request }) do
    draw :subdomains
  end      
end

But this produces this error when trying to boot the app :

/Users/mycomputer/Sites/my_app/config/routes.rb:3:in `instance_eval': You should not use the `match` method in your router without specifying an HTTP method. (RuntimeError)
If you want to expose your action to both GET and POST, add `via: [:get, :post]` option.
If you want to expose your action to GET, use `get` in the router:
  Instead of: match "controller#action"
  Do: get "controller#action"

I don't get the impression that Rails understands what I'm trying to accomplish. Anyone have an idea how I can circumvent this?