I'm going to engage in sexual acts with a guy in a long term relationship
Why do you want to encourage cheating like this?
I want to see how it feels to be with another guy
Then you should pick a man who is single, you are being inconsiderate and disgusting for encouraging cheating like this. The unaware partner will be so devastated and you will ruin all the respect and trust that went on between them.
It is not my problem. My lover is confident that I never do it on purpose and I am honest with my feelings.
While your lover might not have a problem, theirs might and you should think about how they feel. You would not like it if somebody did something to hurt you and just didn't care.