Is masturbation really a sin?

i'm pretty religious, and i'm happily married. unfortunately my job takes me far away in bot time and distance to my wife. there have been numerous times where i've been propositioned, thankfully my faith stands by my wife and i'd rather die than be unfaithful. so, masturbation is the only alternative. the issue is that i was brought up believing that self-pleasuring is a sin. is this really true?