Is it normal i dont mind sharing someone

Hokayyyyyyy so obviously its not normal but i wanna hear other people's opinions. Recently, this guy started to talk to me and hes being real about what he wants to do which is hook up with me cause he hasnt hooked up with anyone for a while since he has a gf. Yah hes probably using me but what if the gf didnt mind? Instead of getting mad, she wanted a threesome. Not just a threesome for one night, but almost, kind of like a relationship of three people instead of the norm two. Even though i know this guy for a while, we do have chemistry ever since, and i would like to have something with him and only him, I actually dont mind if I shared him with the gf. This isn't the only time it has happened. I have hooked up with someone else's bf for a night, only cause it was our chance to do what weve been wanting to do forever. He wanted to continue and we did for a little bit but since he did have a gf and we did like eachother kind of, it would bother me he was still with his gf, but it didnt. I didnt mind him still with his gf cause i mean i was still getting what i wanted out of it, we had a special thing going on. I noticed alot of people are doing this now, dating men/women even in marriages. What makes them or us not mind that the person we like and care for is in another relationship and doesnt mind? With the recent dude, im trying to not do it again,but i dont know temptations are pretty strong and sharing is caring lol.