I think that it's been established that Brian, as well as Michael, are capable of changing their behavior. Back in the first season, we'd never have believed that Brian would be able to have ANY type of relationship not based solely on sex (with someone who's not Michael). Fast forward to S4, where Brian and Justin are having just such a relationship. As much as I dislike Justin, I have to admit he accomplished a previously thought impossibility in getting Brian to commit (even if it was an unorthodox committment).
That being said, I think Brian and Michael could definitely have a relationship - at some point in the future. Because you're right, Michael is more focused on monogamy and commitment, and Brian is more interested in an open type of relationship. But they've weathered eighteen years of change in their relationship, and I think if both of them really wanted to make a romantic pairing work, they could do it.
All things are possible when it comes to love, and we've heard time and again that Brian and Michael do love each other. If they want it bad enough, they can make anything work.
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That being said, I think Brian and Michael could definitely have a relationship - at some point in the future. Because you're right, Michael is more focused on monogamy and commitment, and Brian is more interested in an open type of relationship. But they've weathered eighteen years of change in their relationship, and I think if both of them really wanted to make a romantic pairing work, they could do it.
All things are possible when it comes to love, and we've heard time and again that Brian and Michael do love each other. If they want it bad enough, they can make anything work.