If you install php7.0 without preinstalling either php7.0-fpm or php7.0-cgi, the libapache2-mod-php7.0 package will be installed automatically. That would be bad.
But, you don’t need to install php7.0 package at all! Our documentation about installing PHP does not include installing the php package, and so this dependency never comes up, if you just follow our documentation.
In short: Just follow our documentation, and multiple PHP versions will Just Work! If you don’t follow our documentation and instead follow random BS on the internet that leads to you installing mod_php, either directly or via dependencies, I will be mad.