I guess i don't understand the question.
First, in a monogamous situation (which I was for 9 years) I would never, ever ask my husband not to bee platonic friends with a straight woman. That seems silly if you trust your partner.
Second, I guess I don't understand the big deal here. Parents have platonic friends. You kids know them. If parents have lovers who have been around for a while, it seems like you can a. tell kids they you are poly, talk about the person first and then introduce. or b. just introduce as a friend. My family has has tons of friendships and romances end. My son isn't overly attached to these people - he's 11 and running is own relationships.
Is the play you are coming from the very traditional model which focuses on a nuclear family?