I want to transgress sexually

I'm going to come from a perspective as a Christian, virgin, single, and someone who has had a PMO addiction since 2006 when I say this; parts of this may make you go "oh well it's easy for you to say." And that's fine. This is just my opinion.
I'm going to go out on a limb here and assume "sexually transgress" means "cheat on my fiance." In my opinion (which is worth absolutely nothing and you can take it or leave it as desired), any desire to cheat on your fiance is wrong. This is the woman you're going to spend the rest of your life with. Your bride should be the woman you desire. Not some anime chick (who isn't even real), not some buxom porn star you can never have. Your wife should be the apple of your eye.
The reason you're having these urges is because your brain has been rewired. Fapping releases endorphins, a chemical that makes you feel good. Your brain wants you to feel that good feeling again, and so will throw the urge out whenever and wherever possible to make you give in. In short, you have an addiction. To make matters infinitely worse for you, the human brain craves novelty, which is why porn is so alluring- you can fap to a different girl every single day. Even if you can never have them, the fantasy is enough to give your brain the rush.
You've found a girl who's willing to spend the rest of her life with you (making an assumption here, I know.) That's more than what I have. Treasure it.

 

No problem.
Bearing in mind I'm just an armchair sexologist, I think the problem is your brain is craving novelty. Something I noticed in my own porn habits was that, to get the same rush, I'd look for more bizarre and extreme porn- stuff that would've horrified me became erotic. In my own situation, though, it was largely Pavlovian in nature and I was able to get away from it.
I get it, though. You're eager to explore the limits of your sexuality. But if you've been PMO'ing your brain isn't functioning the way it should be. If your brain is anything like mine, distracting your brain with something intense (video games and hard exercise are popular with Fapstronauts) will cause the urge to subside. If you can keep it up for a month or so, you should find that your desires are going back to what is considered "normal."

 

You should not deprive yourself of valuable experiences while you are young. If you want to experiment then go ahead. If something is holding you back, then don't rush into it or just simply abstain. Just try to think if your actions will have repercussions - and if you don't like the possible outcomes, then don't do it.