SEO and page with checkbox before accessing site

I am working on a Ruby on Rails project in which people who visit the site MUST check a checkbox in order to access the whole site (after checking it a cookie is set to to not show the first page again).

I made it working but my interrogation is about SEO: how google, bing or any robots can reference my website if they are not allowed to go through the page with the checkbox ? Is there a way in Rails to skip this first for some robots ?

Thanks for your answers