The main reason is because of lust. Lust is a sin, and it's hard to masturbate without lusting. You can't masturbate and sing "Amazing Grace" at the same time; it just dosen't work. And even if you didn't look at porn while you masturbate, you still are thinking and lusting with your mind. Also, you are taking pleasure away from your wife or future wife, and that is only supposed to be pleasurable with a married couple.
OK, first thing, you need to use punctuation. I've fixed this post for you, in my quote of it. Now, let's tackle these issues one at a time...
Lust is a sin, and it's hard to masturbate without lusting.
It is difficult to accomplish this, that much is true...but that doesn't mean its not possible. Lust can be born in the heart, regardless of whether an individual is masturbating or not. The two are not inherently linked, contrary to popular belief.
You can't masturbate and sing "Amazing Grace" at the same time; it just dosen't work.
I know you're probably joking with this comment, but I'd like to address it anyway. the core question for every person is, "Why are you masturbating?" Is it to fill a void that needs surrendeirng to God? If so, then you're doing it for the wrong reason. Lust is often involved with this activity, but not always. If someone claims they are able to masturbate while focusing on God (and yes, even if reciting "Amazing Grace" in their mind), who am I to say they're lying? Only God knows and understands the depths of the human heart.
And even if you didn't look at porn while you masturbate, you still are thinking and lusting with your mind.
This isn't necessarily true. I do agree that using pornography for this activity is wrong, but the absence of it doesn't automatically mean that the individual is lusting, either. Also, its important to note that not all sexual thoughts are lustful. If a person masturbates for the purpose of gaining a physical release so they won't sleep around, is it wrong for them to enjoy the emotional "high" that accompanies that release?
Also, you are taking pleasure away from your wife or future wife, and that is only supposed to be pleasurable with a married couple.
The Bible says that fornication, adultery, homosexuality, incest, and bestiality are wrong. All of these are sexual acts which involve some kind of partner, and also violate God's created institution of marriage. As such, they are off-limits, but there is no mention of masturbation in Scripture, one way or the other...it's not in there. It might actually be easier if God had given us explicit instruction on this, but He didn't. This presents a challenge for those who think its sinful, because in order to claim such a thing, they must first explain why God didn't mention it at all. Surely, He knew what would happen in modern society, so if it were inherently sinful He'd have said so.