I found two things interesting about your post. One you mentioned that "I am not gay, Thank God".... To that I say, that you are probably gay or bisexual. I say that in a loving way. That would be the definition of SSA and you should use the resources to help you with that.

Second, If I got into a Mikva with a bunch of naked girls, that would not be a trigger, it would be an explosion !!!! I could not handle that, nor should you be expected too. My advice is not to go into the mikvah at all. HaShem will understand. Why would you put filters on your devices, come to GYE looking for help, read all the books, go to therapy and all the other things we all do t help us with this problem only to jump into a body of water with a bunch of naked people that you get aroused from? You have to give yourself a chance to succeed. 

Best of luck, please keep us posted on how you are doing 

YES, YOU ARE NORMAL. During your teenage yours it's very normal to get turned on very easily, it happened to me too. You have a healthy body, you just need to learn how to channel your energy the right way. I highly recommend you opening up to a Rebbe who can guide you and touch base often.
As you grow up and get older things will somewhat sort themselves out as long as your not watching online filth. 

There are lots of resources here how to deal with urges, check them out. Perhaps check out f2f program too!

You are so normal! 
2 points.

1- We all have desires. Some more "regular" and "accepted" and some less "regular" and "accepted" Its nothing to be ashamed about! Hashem gave us our desires! Even all the weird ones! Can we all stop fooling ourselves? If all the people that feel "feelings" in the mikveh wouldn't go, then basically NO-ONE would go to the mikveh. Unless you have very low testosterone levels. 
You don't have to be 'Bi-sexual' in order for your body to start feeling feelings around 10s of naked people regardless of gender. Our brains are complicated. We can be fueled by curiosity too, or parts of us that are young and scared, or awkworded out. Our bodies work in wonderous ways and you are 100 percent normal. That being said I'm not saying you are not bi-sexuel or gay (I have no clue) I'm just saying if we'd all be a little honest we'd realize that the mikveh experience is exactly how is sounds, 100s of naked men, -big, small, fat, skinny, cool, old, all together in close proximity. In such a scenario a whole range of feelings are normal.

2- Feelings are usually not in our control. What we do control is our reaction to our feelings. You are not a bad person for getting 'heated' and feeling like you do in the mikveh. You are responsible though for your actions.  Gedarim are definitely a way to act in the correct way.

Obviously if you have acted before on your feelings, then you know you can't rely on yourself. If not, take a deep breath.
Your normal. As long as you do no action your ok! Hatzlacha!

wow "chancy" thanks for commenting i will try this and hope to make it  thanks a mill 

I will share somethin lese that might be of help to you and others. 
I now know without a doubt that for vast majority of times i get aroused in mikvah its because im afraid that i will get aroused......
meaning, most of the time, i will go in like anyone else wanting to purify myself and get out. But then i see out of the corner of my eye someone that im afraid will trigger me, (without actually knowing is that guy is an old fat pig?......) but my brain right away goes into hyperdrive, "I CANT GET AN ERECTION, DONT GET AN ERECTION, THERE IS NOWHERE TO HIDE". but that of course makes me get an erection because its impossible not to think about a pink elephant when told "do not think about a pink elephant........ 
So for now, Ive gotten into the habit of not looking at anything and limiting my eyesight to my immediate self, because i know that ME MYSELF & I are NOT interested in anything or anyone. I DONT want anything to happen! I DONT want to think about anyone elsre, so why make it harder, this way i dont see anything and im in and out in no time.
The first step is being 1,000% sure that YOU DONT WANT THIS! if you are still questioning that, this will not be too effective. I amde a long list of reason why this is completly against everything i want and why it will never happen, so now im not fighting myself, but rather a past brainwire that is still stuck somewhere.
Good Luck

Very clearly

wow, I also try not to think of this when going into mikvah, and, and sometimes it does help  BUT sometimes it doesn't help, and the second i step in and i saw what i saw i cant control any more
so im saying here that your tools will only work by me, when im in control of myself, etc, otherwise i get stuck!