Eye sex can only happen if your partner has lost at least one eye. You insert the penis into her/his empty eye hole and fuck it. You can also do with a healthy person with 2 eyes, but this would be very painful and could result in severe eye damage, so it is not recommended.
James: Hey Ian, so I heard your gf lost an eye in an accident! This is horrible! I feel so sorry for her.
Ian: Nah man, its alright, we can now have eye sex!
Merlin: *Stares at Arthur*
Arthur: *Stares at Merlin*
Gwen: This is my husband Arthur, and this is my husband's boyfriend Merlin.
Merlin and Arthur: *Continue with the eye sex*
When you maintain eye contact with a stranger, in an oddly sexual manner, perhaps at the grocery store or driving down the street... for more than five seconds, you have successfully managed to "do" someone, eye-sex style.
Be careful though, don't want to share sloppy seconds with your friends ;) SAFE SEX, wear some SUNGLASSES!