When a woman is so sexually charged she treats a man's penis with an extreme amount of attention for a prolonged period during any particular sex session. The reference is that she's acting like it might be the only penis she has seen, or may see, for a considerable amount of time; i.e., like she's on a deserted island and just found it.
An extremely long head session.
Billy always likes to sleep with fat chicks because they give him desert island dick in appreciation.
Susan loved her birthday present so much she treated her boyfriend to some desert island dick that night.