There's an amazing sort of attatchment between friends- not the love of a couple, sure, but there's this bond and Samwise-esque protection that seems to be a staple attribute of the friend-friend relationship. The more time you spend together with someone, the more this 'claim' of the other person seems to deepen; chances are you'll get into more tiffs, too, but you'll still be closer... Thus, the roommate situation.
When rooming with someone, seeing your fellow get into a romantic relationship with someone else can be quite trying; it's an unconscious and non-malicious (and most usually non-romantic) sense of ownership, where it hurts to see the other's center of attention seemingly shift off of you, and onto another... And the attention usually seems much more meaningful, stronger. It obviously isn't, seeing as it's simply a different type of affection, but the unconscious mind can feel seriously threatened by this newcomer coming in and replacing their spot; you suddenly 'don't mean as much' to your friend in your mind.
Chances are, you suddenly feel as if you're not worthy enough, and as if you're a burden on their relationship; and I speak from experience. Sure, this is an unfounded feeling, but it's there. The immediate reaction is then to back off from that friendship and claim a new one with someone outside the circle, so you don't feel left out. Even if its not meant to hurt the other party, you need to retaliate in some manner so you don't feel overwhelmed by the whole shenanigan, and so you can downplay your own sense of unimportance.
I've experienced friends hooking up aroud me several times now, so I've built myself a sort of immunity to the whole situation- to the point that I was totally cool when Chels hooked up with Wes... And still am! The thing is, in the long run friendships'll last above all- try not to feel left out if someone gets into a relationshp; just talk your way through it, and respect the time they may need to spend with their new hubby.
Just make sure they don't start ignoring you, of course. :)