Your roommate may not be totally wrong, but make sure you speak to the Lady in depth about it. I am not polyamorous, but a good friend of mine is. And I have seen a few other people try to convince themselves that they could be polyamorous. The key to any good relationship is communication. When you enter a polyamorous relationship, either as a muti-person unit or as a couple with some open rules, you now have to be sure you're communicating with EVERYONE. More persons involved means more chances of someone feeling left out or feeling suspicious of un-agreed upon behavior. Yo have to communicate and discuss relationship expectations very early on, or chances are someone will feel like they're being mistreated. A lot of younger people entering poly relationships think that it's just a way to sleep around a lot without actually cheating, they don't realize that they're going to get jealous. Jealousy is a MAJOR thing that will bitch-slap you even harder when you're partner doesn't feel guilty being with someone else. Even when expecting it, it's a hard thing for a lot of poly people to deal with. Just a few things I've noted looking in from the outside, so take it with a grain of salt.
I personally have encountered poly relationships before, both first-hand and second-hand, and my biggest conclusion was that I was not willing to spend the time required in order to pursue more than 1 romantic relationship at a time. I am big on having time to myself, to do my own thing, to just be by myself and do whatever I feel like at the moment. When I date someone, I usually like to see them every 2-3 days, or roughly 2x a week minimum. To date 2 people, assuming I already know both of these people and don't have to spend time meeting people/getting to know people/going on dates, I'd find myself spending 4-6 nights a week on dates, or at least, hanging out with each of these 2 people. (Mathematically speaking, of course.) That only would leave me with 1-3 nights to hang out with my friends, not to mention spend some quality time with good ol' me! When was I going to get any reading done, for instance? Or writing or book-binding (just throwing out theoretical activities at this point)? Or even just vacuuming, dusting, feeding my cats and taking out their dirty litter? That has always been my fundamental issue with the poly lifestyle; if I am interested in someone, I want to see them more than once a week. But I don't think that scales at all when you throw in even 1 more person into the situation. You either spend less time with each individual than you'd like, or have no time to yourself/your social life outside of dating.