Dude...ugh...i cant speak with a person who thinks hes sinning by stroking his reproductive organ...
The only reason they think it's sinning is because it's a so called "waste of sperm" It's a long-standing part of most christianity that masturbation is a BAD THING, primarily because it can be percieved as a "waste" of sperm. In other words, because you're not getting someone pregnant, you are wasting your precious seed. Thing is, sperm have a lifespan of only a few days anyhow, so every time you go for a week without sex, you are, in fact, "wasting" millions of the guys.