The term sounds glamorous, and I've always scoffed and laughed at how soap operas treat affairs, but now I think I might be a victim of my own premature judgement... So...if I, as a single guy, has sex with a guy I know is involved, am I having an affair too? I know it's only a word, but like I said, I NEVER thought I would be on this end of the moral compass...I was always a good kid you know...
Yes, you are taking part in the affair. You technically are not the one that is in a relationship but you are the 'Mistress/Mister' in this matter. It is best to not get into that type of thing unless it is open relationship.