The best way to release sexual urges can be found at the ends of your wrists.
but wait, erin! I hear people saying. (And then I think maybe I've taken too much cold medicine.) Masturbation is fun, but this guy is really hot! And sex is different with a partner!
Well, sex is sex. Attraction is attraction. One can be in a relationship with someone and really really want to snog the cutie across the street. It happens.
However, the question, as always, is what you do about it. My opinion about it is this: if I have made a commitment to someone, and that commitment includes sexual exclusivity, then I do not act on my attractions to other people. I still have them, I still might fantasize about them, but I don't act on them.
So you have some options if you are attracted to someone else while dating someone else.
1) Be attracted to the other person and want them and not act on it.
2) Be attracted to the other person, want them, and decide to break off the committment with your current boyfriend to pursue this attraction.
3) Be attracted to the other person, want them, and act on it. If you act on it without telling your partner ("cheating") and break the committment you have to them, it will most likely end the relationship.
4) Be attracted to the other person, want them, and decide to re-negotiate the agreement with your partner so that you're no longer sexually exclusive. This is tricky. This is difficult. This requires a lot of communication, and it also requires your partner to be willing to go along with this.
So, it's up to you to decide which of these options works for you. Personally, I've found option three to be pretty lousy for all concerned, especially if you purport to love someone.