Defines a type of person whom you might really be into, but is only interested in you long enough to have sex with you, and drops out of your life immediately after, looking for the next bang. Can be used to describe dudes or chicks.
Jack: "Last night was amazing. How about your number so we can do this again sometime?"
Sally: "Sorry...I'm what you call a "BYEsexual". Once I have sex with you...BYE!!!"