I don't think there is really a difference between masturbation and not-procreative sex from a metaphysical point of view. The first is amusement, the second is amusement.

Masturbation tends to be condemned very often by various environments. But why? How is it different from not-procreative sex?

The case is different with procreative sex, though. It is free of any sin because the intention is pure. Christian would agree with this statement.

Let's take this forth. If it is the intention that decides, there is also a second way of "pure" sexual act. It is a sexual act of love. You make it from love. The teachings of Yoga state that sex without love is pointless. I take this further. If non-procreative sex is the same as masturbation, then masturbation done from love (we can take your example of visualizing someone you admire) is also meaningful.

This metaphysical or philosophical thoughts could be used as an excuse for masturbation. This is not the point. I wondered why masturbation was condemned in the beginning, right? Sex is a natural thing that occurs between man and a woman. Masturbation isn't natural in that light, because a hand/a tool is just an artifical replacement of the other lover. This is the answer to my question.

Do not take Yoga too seriously, do not be bothered by the philosophical thoughts. Everything that is done moderately between extremes is right. Masturbation is only slightly different from sex (philosophically), and if sex isn't bad, masturbation isn't either. It is not a bad thing to do because you do not harm anyone; it is not a bad thing to do unless it isn't an addiction.

Everything is about what you believe in or not.

It can be dissipating; physically, mentally, spiritually if over-done. It can also become too consuming...taking up too much time, too much thought, too much planning, too much guilt. But I don't think it's a base act any more than any other type of sex unless you were to somehow become unbalanced with too much of it. Otherwise it is just an occasional release.