Is sexual energy real (biological and physiological), or is it purely psychological?

Both: At your age most males have a very health sex drive owning to the surging testosterone. It is still a component of psychological stimulation as well.

Physiological and..: Sexual desire/energy has a strong physiological basis. The survival of all species that reproduce sexually depends on a strong innate sex drive or that species would die out. Hormones like testosterone and estrogen have a strong influence on sex drive and function. Obviously there are also psychological factors that may increase or decrease sexual energy, but the basis is strongly physiologic.