I have some awkward questions. The Tanner Stage Calculator says I'm in stage three but I haven't really hit my growth spurt, at least I don't think I have. I'm 5'3" and I'm 13 years old. Also is it normal to get erect when seeing other guys' penises? And is my penis length normal? It is 4 to 4 and a half when erect and usually 3 when flaccid. Also is my weight normal? I'm 118 right now. I hope that's not to many questions! Thank you !
Let's start with the last question first. At 5'3" and 118 pounds, you have a Body Mass Index (BMI) of 21 which is right in the middle of the healthy range. At your age about 70% of your peers are skinnier than you are, but that is because all of you are right in the midst of your growth, which consumes a lot of energy. A lot of guys tend to get too skinny during this time, but then catch up later.
Concerning the Tanner Stage Calculator, if you get a 3.0 as the estimate, that means you just entered stage 3. Your growth will occur during stage 3, so you are probably about six months from starting your growth spurt. Typically you get a score of 3.5 when you are in your growth spurt.
Your penis length is normal to a little longer than average for someone in stage 3 of growth.
Now, for the awkward question. Yes, it is perfectly normal at your age to have erections when seeing another guy naked. When you first gain the ability to have erections, they are very hard, if not impossible, to control. It takes a while for your brain to figure out when is the right time to trigger an erection.
One of the problems you face is that erections are triggered when a small muscle inside you clamps down on a vein. This keeps the blood from leaving the penis at its normal rate. The result is that the penis swells as more blood comes in than goes out. Typically when you see another guy naked, it is usually in a situation where you are also naked. You're nervous and that tension triggers an erection.
Another problem is that with erections comes the concept of sexual feelings. You have at least a vague understanding of what the penis is for and seeing them starts your brain down the path of thinking sexual thoughts or at least sexual feelings. The brain responds by sending a signal to your penis to prepare for sex. The problem is that it is not the right time, place, or person. That gets you feeling awkward, nervous, and -- oh, no! -- your erection gets harder.
It will eventually calm down and your own reaction to the situations ("No, not now!") trains your body in when not to have erections. A lot of boys are told that having erections around other guys means they are gay. It has absolutely nothing to do with homosexuality. It is simply a phase all guys go through as their bodies learn when to respond and not respond to various situations.