Redditors in open marriages, how did you pitch that idea without it erupting into a huge fight?

During sexytime, see if your partner is open to a little fantasy and ask “So, if we could have someone else here with us right now, who would you want?” And, gauge their answer, and then you can kind of pursue it from there. If they play along, sometime later, maybe something like “Well, that’s hot. If you and so and so were here, could I just watch? That’d be hot.” And so on.