Look at it this way: how many mono married (or long term partnered) couples do YOU know where you get along equally well with the guy and the woman? Or if they are gay, both guys, both women? When I think of couples I know, I almost always prefer one or the other. Sometimes I LOVE one of them and can hardly stand the other.
Usually, I prefer the woman, but there is at least one case where I love the guy but can't stand his wife. I've had to stop visiting them because I can't even have a decent conversation with the guy without his shallow needy materialistic wife barging in with non sequitors.
So, with romantic relationships, it's like this, only intensified. Sometimes a triad will work for a while, when new relationship energy is in place, hormones and sexuality are raging, but once that fades, the true affinity may settle on one or the other of the couple.
Most polyamorous people date separately.
Love withers under constraint; its very essence is liberty. It is compatible neither with envy, jealousy or fear. It is there most pure, perfect and unlimited when its votaries live in confidence, equality and unreserve. -- Shelley
me: Mags, 59, living with:
miss pixi, 37