You don't have to embrace BDSM if it makes you feel too uncomfortable. Just allow for them to have their fun and negotiate a way so you don't worry about wandering into it.
But it sounds like you do want to explore it so you can deal with it better. Maybe watch them doing much lighter BDSM and see how you react. Let them know how you feel. See if you can appreciate the consentual aspect of their play and focus on that.
I don't think therapy is a bad idea, but I wonder how many therapists would blame polyamory as a problem.