Okay... here's how I see it. Having a heart-to-heart with your so or spouse about your polyamorous orientation is one thing... 'coming out' and saying that you wish for the relationship to go in the direction of polyamory and asking for input, etc. It can go either way.... A) so/spouse is accepting of your orientation and understands that you love him/her, and want to have relations, serious or not, with another (or others). You set boundaries, terms are made very clear, and you're both on the same page about how the relationships progresses. In this scenario, it is acceptable (to your partner) to proceed. A relationship orientation is established, and proceeding is not cheating. B) so/spouse is not accepting, and feels that your orientation is not something he/she can live with. In this scenario, a decision is made.... either end the relationship, due to orientations clashing, or proceed anyway. In this scenario, proceeding is cheating.