Sometimes I sleep naked. When I wake up on my stomach and move around just a little bit I ejaculate. Is that sinful? Also, when I'm sleeping naked, is it sinful to sleep on my stomach? Because when I move a little I fell a weird sense of pleasure. Is pleasure a sin?
There is no law saying how you must sleep or what you must wear while sleeping. What you are doing by rubbing your penis against the bed is masturbating. By itself it is neither right nor wrong. In one sense, ejaculating semen is merely a bodily function that all men have. A part of that function includes an intense sense of pleasure that is called orgasm. It is just how God made men.
However, Satan is out to sell sin and he uses our desires to make sin look appealing. "But each one is tempted when he is drawn away by his own desires and enticed. Then, when desire has conceived, it gives birth to sin; and sin, when it is full-grown, brings forth death. Do not be deceived, my beloved brethren" (James 1:14-16). Personal pleasures can be so distracting that we forget to look to see whether what we want to do is actually something good or bad. Satan distracts us with pleasures so often, it is easy to just assume that if something is fun, then there must be something wrong.
God built our bodies to feel pleasure to encourage us to do things necessary for life. We have to eat and drink, so when we eat or drink, there is a feeling a satisfaction. We need to have children for the next generation, so with ejaculation comes a feeling of pleasure. But just because something is pleasurable, it doesn't mean we can do it any time we feel like it or in any manner. We get pleasure from eating, but if we eat all the time, we get fat. If we eat junk food constantly, we lose our health. If we drink products with alcohol in them we don't think reasonably and eventually harm ourselves.
The same thing happens with ejaculating. If we start chasing the pleasurable feelings that come with ejaculating, we start making bad choices. Some guys rub their penis so much trying to ejaculate that they actually rub it raw. Some start looking at pornography because it makes them sexually excited and more likely to ejaculate. Others start fooling around with sex long before they are ready to get married. Each of these involves bad moral choices, but the boys don't think about it because they are focused on wanting the pleasure.
When you are doing something that is not wrong, there is nothing wrong with enjoying what you are doing. "So I commended pleasure, for there is nothing good for a man under the sun except to eat and to drink and to be merry, and this will stand by him in his toils throughout the days of his life which God has given him under the sun" (Ecclesiastes 8:15). It is perfectly fine to enjoy your supper. It is fine to enjoy a cool glass of water after running around outside.
Your body produces semen all the time. It has to because semen is only good for a limited amount of time. It is stored in the seminal vesicles, where it is made, but the seminal vesicles only have so much room. So when the the seminal vesicles get full, you get an increasingly stronger urge to ejaculate, though you might realize what the urge is at first. Eventually you will have a wet dream or you end up masturbating. That gets rid of the excess and the cycle starts all over. That is normal and there is nothing wrong with the fact that it is enjoyable.
But when you start pursuing it, then you open the door to temptation and sin. "For this is the will of God, your sanctification: that you should abstain from sexual immorality; that each of you should know how to possess his own vessel in sanctification and honor, not in passion of lust, like the Gentiles who do not know God" (I Thessalonians 4:3-5). If that went over your head, here is what Paul said: God wants you to keep yourself set apart for a special purpose. Your body belongs to your future wife, so don't go polluting yourself by having sex before you are married. You should know how to control your own body, treating it as something special and honorable. We are not to be like the rest of the world who don't care about God and so chase after sexual lusts and passions.