I am 18 years old, and an Asian boy. As I grow older, I worry about my future in regards to having a sex life with my wife. When I'm erect my penis is only 8.5 cm (3.5 inches). I worry that I won't satisfy her and give her kids. Is there any chance I might have a micropenis because my friends always tease that I have a small penis.
The normal range for a male's erect penis is between 3 inches (7.5 cm) and 8 inches (20 cm). Your penis is not considered a micro-penis. A micro-penis is when the length is less than 1.5 inches when erect. Yes, three inches is less than the average Asian penis length of 5.2 inches, but it isn't unheard of.
Having a penis that is three inches long will not effect your ability to have sex. The typical female's vagina is three inches long when she is not sexually aroused. In addition, the wife's body is able to adjust to the size of her husband's penis within a few months of marriage. Since you are smaller than average, your wife will become used to having sex with you faster than a man who has a large penis. The sensitive areas for a woman are in the first inch or so of the vagina, so man with smaller than average penis size can still satisfy his wife. Most of the pleasure in sex comes from how it is done and not from the size of your penis. There may be some sexual positions you cannot use because you just won't have the reach, but it doesn't mean all positions will give you problems.
Nor will you have to be concerned about your ability to have children. The typical male shoots semen about six to twelve inches when he ejaculates, which is more than enough to reach your wife's cervix. In addition your sperm swim, so even those delivered only part way will make their way to the fallopian tubes to cause conception.
Whatever your size, you should be happy with the way God made you. But in your case, I believe you are worrying over something that will not be a problem -- unless you make it into a problem.