It is definitely acceptable to ask first. By asking him for permission, you are actually showing him that you respect him and love him enough to be open about these things. My wife had a similar situation several years ago and just went ahead and cheated on me. It broke my heart and hurt, but we eventually got past it. About 2 years later she wanted to try it again and asked this time. I told her it was fine and I felt the bond between us stronger than ever. Sometimes it really is just a quick fling to get it out of your system and that doesn't have to mean cheating or the end of a relationship. Worst case, he says no and ends things. If that happens though, he just didn't have enough love for you and you can't blame yourself for doing nothing more than being honest with him about your feeling/desires.

How about you don't fuck the other guy? How about you stay loyal to your relationship? Is sex with another really worth the damage to your partner? If you lie and cheat, even if your partner never finds out, you will know what you have done and you will know that you are a liar and a cheater. You will have to live with yourself. This other guy cannot be that great if he is chasing after a woman who has been spoken for. He is an opportunist, and you find this attractive? You may need help with your reasoning. 

If you are going to do this, let your partner know, but be fair and give him his options. You are going to fuck another guy, he can live with it, or he can leave. He should know that you care so little about him that your orgasm with another guy is more important than his feelings. If he stays, let him know that it is okay for him to fuck around too. Let him know that you will now have an open relationship and there will be plenty of other people in your bed with you. 

You are either retarded, poorly raised, or a selfish fool - tell your boyfriend and set him free, he is better off without you. 

Fuck him then ask ur partner, if he agrees then fuck him again.