Paul David, I might echo what everyone else has already told you, it's okay to be who your are and to do whatever makes you happy. However only part of your question has been answered. Now what I am about to say is being based off the thoughts that you are hetero and not gay. If you are gay, which is perfectly fine then the easy and logical answer is that you envision yourself in the med fet scenes because you are attracted to guys. And the reason you were attracted to guys when you were going through puberty is because you are in fact gay.
However, assuming you are not gay, JDS4Med, a guy that I have found to be right on track and usually offers perfect advice hit the nail right on the head concerning why watching guys in a med fet scene is a turn on for you. It is because you can see yourself as being the one in the scene. Now as far as you being attracted to guys during a year you were going through puberty, well I can offer you a simple and logical reason for that as well.
During puberty people often find themselves attracted to the same sex. Psychologically speaking a lot of the time this occurs because your hormones are raging. There are so many changes going on and so fast that it's a very confusing time. Most people are a bit confused about what's going on as well as intrigued. You have heard the stories and maybe even had a little sex class in school, but even with that information it's still quite weird. So for a lot of guys, and it seems this happens more with guys then girls, your attraction towards the same sex isn't always out of being attracted per se, but rather trying to figure out what the heck is happening to you. You basically are longing for information confirming that what you are feeling during this time is normal. And you can't ask a girl if what you are feeling or what is happening to you is normal or not. So because you are so unsure of everything, the last thing guys want to do is put themselves out there in front of the female gender and then crash and burn. So, deep down you know that by sticking with "like" things you won't have to put yourself out there until you are more comfortable with the changes that are happening to your body. You know what they say, there is safety in numbers, lol.
Now the other possibility is that you are bi- but I honestly don't think you are because you said you went from liking girls to not liking girls for about a year, then returning to liking girls again. Which supports my theory as mentioned above.
Either way you spin it, or no matter what conclusion you come to, at the end of the day you have to lay down with you. And if you can lay down with yourself and be okay with it, then it's all good.
Mashie