An explicit agreement is only needed when you need the user to relinquish any rights they would usually have.
Open source licenses are usually not designed to take rights away from the user they already have. They give them rights they would usually not have, namely the right to modify and redistribute. When the user does not agree to your license, then "all rights reserved" applies and they are not allowed to copy it at all.
So to merely look at your website, you do not need to bother the users with your license. You giving them the right to view the website is already implied by you putting it online on a public webserver without any access control in place.
However, when you want users to submit content under your preferred license, you need to make them aware of your license terms, because they are giving up certain rights to their work. Forcing them to confirm that they understood that "all your contribution will be licensed under [LICENSE]" is important to avoid any misunderstandings.