I'm curious I don't think that masturbation is mentioned in the bible (and being agnostic I don't care) but it's pretty clear that what one is imagining or fantasizing about is important.

Matthew 5:27 and Matthew 5:28 are the twenty-seventh and twenty-eighth verses of the fifth chapter of the Gospel of Matthew in the New Testament and is part of the Sermon on the Mount. These verses begin the second antithesis: while since Matthew 5:21 the discussion has been on the commandment: "You shall not murder", it now moves to the commandment: "You shall not commit adultery".
In the King James Version of the Bible the text reads:
27 Ye have heard that it was said by them of
old time, Thou shalt not commit adultery:
28 But I say unto you, That whosoever looketh
on a woman to lust after her hath committed
adultery with her already in his heart.
The World English Bible translates the passage as:
27 "You have heard that it was said,
'You shall not commit adultery;'
28 but I tell you that everyone who gazes at a
woman to lust after her has committed
adultery with her already in his heart.

I'd assume that applies to both genders and all sexual orientations
I'm not sure why anyone would believe that because the bible was written thousands of years ago that they can change or reinterpret it to conform to modern day standards. Seems like it would be up to god to take care of that.
I think it's up to the individual to decide by his/her own moral code if masturbation is "wrong." If you masturbate and it's causing you distress then you may want to talk about that with a therapist and try to understand why.
If it's any consolation to you most Christians pick and choose what they want to believe and follow in the bible. I don't know a single Christian who follows the book of Leviticus. Some say it doesn't apply to Christians but nobody has ever been able to explain to me why it doesn't.