When I'm around girls, everything just feels natural. I can talk to them just fine. I don't have any dirty thoughts about them, and I know for a fact that I can have a good time with my lady friends.
The thing with guys is that when I'm around them, everything shuts down. My brain blanks, my mouth stops, and my eyes are glued to any object that can get my eyes away from them. I'm a 13 year old guy. A lot of people, including adults, say I flirt a lot with girls. I have sisters and a brother who doesn't live with me. I do have guy friends, but with them it just doesn't seem as good as being around girls.
Another thing is that it's way easier to make friends with girls that I just meet than it is with guys. I am a pretty social guy if I say so myself, so I don't think that's the problem. I just want to know why that is.
One more problem is that when I'm around girls, erections don't come up unless something dirty comes up. With guys, I look up, downstairs wakes up, and I'm stuck in my head wondering why this is happening. Why does this happen with me being a guy?
What you describe is simply what you are comfortable with. You grew up in a house with a mom and two sisters. You have a brother, but he's not around. Thus, you have had lots of practice dealing with women and talking with them. I might be wrong, but it sounds like you have less interactions with guys at home.
Add to this that as a guy, you have reached the point of being aware of sexual feelings. You also know that other guys are in the same boat. While girls have sexual feelings, they are of a different sort and more difficult for a guy to directly relate to them. Seeing guys reminds you of those feelings.
Your body is interested in sexual feelings, especially at your age, and it really doesn't care what triggers them. I would suspect that you are still suffering through spontaneous erections, where your penis gets hard for no apparent reason.
One of the triggers for erections is nervousness or tension. When we get tense, we end up tightening more muscles than necessary. For example, when a person gets tense, they often draw their shoulders up without thinking. Stay tense for a while and your neck muscles start to ache. Well, erections are triggered by a muscle that clamps down on a vein at the base of your penis. More blood comes in than flows out, and your penis inflates. Also when you are nervous, your heart rate goes up; thus, pushing more blood into your penis. The result is that it is easy to end up with an erection when you are tense or nervous.
My guess is that you are a bit uncomfortable around guys and not sure how to behave around them. You are used to talking with girls, but guys tend to be minimalists with words, especially when dealing with other guys. That doesn't give you much to work with in launching a conversation. That makes you slightly nervous, which has led to erections. That just increases your embarrassment. Now you are nervous that your body might respond when you are around guys, which leads to a response, which makes you more convinced that something is wrong.
Understand that all guys have problems with spontaneous erections. The best way to deal with it is to ignore it. I doubt other guys will notice unless it happens to be a huge one. Even if someone does notice, make a lighthearted remark, such as "I guess it's wanting some attention" and go on. If you make it clear that you are not going to let an erection embarrass you, then the guys who like to tease will find something else to tease you about. Your body also learns that erections around guys isn't the right time and it will eventually calm down.
When you run into situations that are a bit difficult, often the best answer is to force yourself to deal with it. Break the ice within yourself and you'll find other guys warming up and wanting to be around you.