In some cases kissing a female friend is arousing to a guy or boyfriend. However like everything, there is exceptions to the rule and because you mixed a lie up into the pot, things will get a bit more interesting. Depending on the type of guy you're with, he'll do one of two things. He'll either shake it off as if it doesn't matter or he will be upset because you lied to him. A lie usually signifies that you're trying to hide something from him and if you personally don't think it is a bad thing, why would you hide it from him in the first place? Being secretive about it will only cause a problem and if he finds out from someone other than you, the situation will inevitably get a whole lot worse if your boyfriend is a moral person and cares about you. If he shrugs it off as if it means nothing, maybe its his way of ignoring the "I kissed my friend" situation because he wants to act like it didn't happen. In other cases he may not give a damn about it because the relationship is nothing more than a physical attraction to him. If you haven't told him and he doesn't know about it yet here is what you need to do.
You need to sit down with him and come clean. Tell him literally "I kissed my friend because ________." I know this isn't the easiest of things to do and depending on the guy, you may not want to try being all cute and buttering him up before you break the news to him. That might incite an even more gruesome reaction out of him when he catches on to you trying to soften him up before admitting that you lied to him. Now if it doesn't bother him and he accepts the apology but tell you to not do it anymore, you should respect it because he did not go off on you. In the eyes of some, the "reason" for you kissing your friend will define whether or not you actually cheated on him so be sure to choose your words wisely.
Now worse case scenario is that your boyfriend has found out from someone else before you could come clean or while you were hiding it. The catch to this end is that when you are confronted with the question "Did you kiss _____?" You need to come clean right away because you never know who the source is that he may have gotten the word from. However just because you tell him when he asks doesn't mean you're out of the woods yet. You still hid the occurrence from him which is never a good strain on a relationship because it can lead to the manifestation of distrust or even lower his opinion of himself. He may feel like you don't trust him enough to tell him the truth which is not good either. When you come clean after being confronted about it, you need to apologize, reassure him that it will not happen again and actually follow through with it. If you slip up a second time and hide it or even come out as soon as it happens, you're still not keeping your word to him which can shatter the trust in the relationship further. If your boyfriend doesn't care one way or the other about what you do, then its not the "relationship" you're looking for. If he wants something serious, the distrust will bother him in some way or another and if you're looking for something serious you need to turn and walk away from this relationship.