Have You Ever Realized (After the fact) That You Were Someone’s “Side Piece?”

If someone already has a partner or married or is taken and they approach you with a casual ongoing friendship with benefits are they not essentially calling for a “side piece” in their extra marital affair? Or how do you perceive this when you are approached with this scenario? I’m done with it, I’ve acknowledged essentially you are second place, second choice. They seem to want you whenever they call upon you but it’s definitely not the same if you call them whenever you want or need them. I won’t be someone’s “side piece” ever again. They don’t tell you this and you get caught up in fulfilling your needs with them and enjoying them but at the end all you cannot develop feelings or more because you, my friend/s ain’t nothing but a side dish on a main course right along the main dish.