Hey Bluebird
I think it's clearly sin... 3 reasons, 1st one just sets it in stone pretty much.
1. Lust is sin... in order to masturbate... you gotta lust.. have sexual thoughts. Matthew 5:27-28
" You have heard that it was said to those of old, `You shall not commit adultery.' 28 But I say to you that whoever looks at a woman to lust for her has already committed adultery with her in his heart."
I don't think we even need to go further, case closed pretty much.
2. Bible talks about unspeakable sins... and considering how not only Christians but even the non-believers are embarrassed about this. It's pretty clear by our own convictions. This is is one of those sins.
3. As far as I can tell... God seems to want to keep sex within marriage... between a man and woman only. Not with animals... not before marriage with another person... so going off the trend I'd wager God wouldn't want us having sex with ourselves.