I feel it's important to note that polygamy and polyamory are two different things.
I don't know how much you know about polyamory, but I'd recommend all three of you researching it before you decide it isn't for you. The aversion to it that you and/or your two friends feel may just be due to a lack of knowledge on the subject. Because society puts so much pressure on people toward monogamy, it's easy to think you're against anything else, but not fair to say so until you fully understand what exactly polyamory entails.
That being said, I suggest you be as honest and open as possible with both of your friends so that you guys can together figure out the best solution to your problem. Honesty is key, and so far it sounds like you're doing fairly well at that, if they both know about each other and the situation.
If you do come to the conclusion that you must be forced to choose one, you have to decide based on what's best for you. Momentarily ignore whatever pleading they may or may not be doing and the possibility that one will be hurt. Just focus on who is better for you in your life. Who makes you happy, who can take care of you, who can respect you, who you can trust, and who you can count on. Write out a list of pros and cons about each of them if you have to. Forcing yourself to be logical is the best way for you to think through the feelings that can be overwhelming at times.