Open relationships
Justin and Brian have an open relationship for most of the series, or at least they do when they’re still together, which is off and on. The reason for this is because Brian doesn’t believe in commitment because it’s for “straight” people and he thinks gay people should be free to have as many hookups as they want. This is actually quite a common consensus, with gay men at least, especially pre-marriage equality.
Some LGBTQIA+ people feel like marriage and the idea of having a family, with kids, pets, and a white-picket fence, is too heteronormative and that it shouldn’t be the ideal for queer people, who aren’t meant to settle down. Obviously, now that LGBTQIA+ people have more rights as far as marriage and children are concerned, this may not be as common a belief, and it’s definitely more subjective. Brian Kinney was one of the people, back in the 2000s, who believed it.
For much of the series, Brian has hookup after hookup with an endless stream of hot young guys from Babylon, the gym, the bathhouse, and basically anywhere else he steps foot, even while he’s dating Justin. Even though Justin hates it, he agrees to it in order to stay with Brian. You can tell early on that Justin is something special in Brian’s life, even if he doesn’t want to admit it.
There are other bouts with open relationships in the series, like when Lindsay and Melanie have a little fling with Mel’s ex Leda. Still, it is most consistently shown through Brian and Justin’s relationship. It’s nice that Queer as Folk chose to show more than just the promiscuous party gays but also balanced out the monogamous, “boring” couples with a more modern relationship.