The human semen is composed mainly of seminal plasma, which is the fluid secreted by the accessory male reproductive organs (the seminal vesicles and the prostate gland). The bulbourethral organs glands also contribute with a small amount of viscous mucus, which gives semen its texture and cohesion. Semen, as thus, is just a composition of body fluids, and washing your hands with soap is a perfectly acceptable and through way to clean it off.

Quick Suggestions:

Masturbate with a sock or glove handy, so that when it comes time to ***, ejaculate into the said sock/glove without making a mess.

Masturbate in the shower or bathroom and *** into a drain. Really easy way to avoid a mess.