The Bible indicates clearly, despite much of what is said in this forum, that certain behavior is wrong, and even when reading the verse(s) in context with the entire Scripture, it remains clearly wrong. Other things must be bootstrapped, sometimes over several iterations, to get to a view. I think masturbation falls into the latter category. If someone feels it is Biblically wrong, then they should not do it, but I can see people making a strong argument that the Bible simply is not definitive.
There is one good set of verses that I think addresses matters that fall into the second category:
Romans 14
I know and am convinced in the Lord Jesus that (23) nothing is unclean in itself; but to him who (24) thinks anything to be unclean, to him it is unclean.
15 For if because of food your brother is hurt, you are no longer (25) walking according to love. (26) Do not destroy with your food him for whom Christ died.
16 Therefore (27) do not let what is for you a good thing be spoken of as evil;
17 for the kingdom of God (28) is not eating and drinking, but righteousness and (29) peace and (30) joy in the Holy Spirit.
18 For he who in this way (31) serves Christ is (32) acceptable to God and approved by men.
19 So then [1] we (33) pursue the things which make for peace and the (34) building up of one another.
20 (35) Do not tear down the work of God for the sake of food. (36) All things indeed are clean, but (37) they are evil for the man who eats and gives offense.
21 (38) It is good not to eat meat or to drink wine, or to do anything by which your brother stumbles.
22 The faith which you have, have as your own conviction before God. Happy is he who (39) does not condemn himself in what he approves.
23 But (40) he who doubts is condemned if he eats, because his eating is not from faith; and whatever is not from faith is sin.
Verse 21 is on point--we should not do what causes our brother to stumble, and in that sense, masturbation may be wrong; however, it appears to be a matter of individual conscience.