One who is Marcossexual will typically only be sexuallyattracted to and in love with Marcos. They often find themselves needing to kidnap and hold Marcos captive in their basement as their new pet, never being able to escape.
When you are in love with Marco Bodt (from attack on titan) so much to a point where you love him and him only and you get "offended" when someone makes a half joke about him aka freckled jesus.