Why is it that whenever I masturbate, and I do so without the use of any kind of porn or lustful thoughts, I always feel guilty afterward and want to stop?
We assume that anything that feels really good must be wrong, especially if it is related to our sexuality. But the main reason actually has a physical cause.
As your seminal vesicles fill, hormones are released that increase your sexual desire. The primary chemical is dopamine. As it rises your interest in sex increases. When you masturbate, these hormones rapidly rise and at orgasm there is a "dopamine storm" in your brain for a brief moment. This is followed by the release of a chemical called prolactin. It, in turn, causes your dopamine levels to suddenly decrease. Without this turn-off mechanism, damage would occur from the too high dopamine levels.
As a side-note many addictive drugs increase dopamine and some interfere with prolactin. You might think such would increase sexual abilities, but the opposite happens. The mechanism in the body burns itself out and one gets to the point of being unable to have sex unless high and eventually unable to have sex at all.
This sudden decrease in dopamine is what makes a guy suddenly feel sleepy after ejaculating. But if you stay awake, the decreased dopamine level induces a mild depression that lasts for about 20 minutes. It causes a sudden lose of interest in sex, which is actually needed to give the body a chance to recover from orgasm and the ejaculation. It is typically called the refractory period.
It is that sudden shift from sexual excitement to no interest and a mild depression that you are interpreting as guilt.