The unused embryos are typically stored frozen, but that said, how does the Catholic church which terms masturbation as “mortal sin” treat nocturnal spontaneous emissions (wet dreams)? This is a spontaneously-produced ejaculation accompanying an erotic dream, a function the body performs when the testicles have no room to store newly produced sperm:
http://en.m.wikipedia.org/wiki/Nocturnal_emission
Mortal sin has three components: it must be a sin of grave matter, it is committed with the full knowledge of the sinner, and it is committed with deliberate consent of the sinner. Thus, nocturnal emissions are not mortal sins.