Sex is easy; relationships are hard.
The heart of the matter is that relationships are about sharing who you are, what you think, what you want, where you want to go. That opening up creates intimacy and a bond between two people. And part of that bond is protecting that trust and intimacy - which is the same thing as protecting your partner.
Here is where this affects you - you have to share your sexual desires and experiences with your partner to build intimacy. And you have to respect her boundaries to protect her and your relationship together. Now you want to add some extra-curricular sex into the equation.
The question is, where exactly are those boundaries? They are wherever you each define them. If blowing a guy falls outside her boundaries, then she is going to feel betrayed. You can never assume where they are, you have to talk about this and figure it out for yourselves. You need to talk about this fantasy and how you want to act on it and find out what it means to her. Is she turned on? Is she OK with you exploring this? Or does this make her afraid that she is going to lose you? (If you want a dick, that is not something she can provide, and that freaks some girls out.)
If you having a sexual encounter with a guy is outside of her comfort zone, but that is something that you want to experience, then you are best laying the cards on the table and being honest about it. Then you will have to make a choice - either give up the fantasy, or admit that this relationship isn't a good fit for you. There are other women out there that would be turned on by that. You want to be with one of those women.
The alternative (and the choice most guys make) is to just sneak around on the side and fool around. But the problem with that is that A.) she will probably find out at some point, and more importantly B.) the moment you start withholding part of who you are and lying to cover things up you begin to kill the intimacy and trust of the relationship - the very reason that the relationship exists. You are locking a part of yourself away - a room that she is not allowed to enter. The moment you do that your relationship is like the Titanic striking the iceberg. There is now a hole below decks that can't been seen but unless it is patched it will doom the ship.
You sound young and figuring things out. I am old and have been around the block quite a few times. I realize that the advice I am giving to be honest with your partner is hard and that the alternative of sneaking around is easy. But from my vantage point, I can tell you that the consequences that will come depending on which fork in the road you take now are completely predictable. The easy road will doom your relationship and the hard road will either save it, or bring it to a close on the most honest and respectful terms possible.
There is a long game to be played here that can reinforce your honesty, trustworthiness, and integrity as a person and a loving partner. Having sex with a guy or not is not the main issue here. As a matter of fact, I am assuming that is going to happen sooner rather than later. But you can experience cock on the down low or with integrity. The main issue is who are you and who do you want to be?
