We all know that Brian Kinney never does anything he doesn't want to and never says anything he doesn't mean. He also doesn't lead people on to believe something that isn't true. This is important to note during his conversation with Debbie about his relationship with Justin. Debbie asks Brian if he loves Justin, and Brian says nothing. Brian would never lead someone on to believe that he loves and cares for someone if he didn't, especially because he puts up this front of caring for no one but himself. But he also wouldn't readily admit that it's true. The fact that he says nothing to Debbie's question confirms that he loves Justin, because he would've ardently denied it otherwise. Now, Brian realizes that he loves Justin (thanks to Debbie!), though it will take him 3 years to admit that to anyone other than himself.
The rules established by Brian and Justin are as follows:
1. Never sleep with the same person twice.
2. No names or numbers exchanged.
3. Always come home by 3 A.M.
4. Can't kiss anyone else on the mouth except each other.
Brian never breaks any of these rules, with the exception of the night he spent in jail, but that's excusable.
Brian also never kisses anyone else on the mouth except Justin (he does kiss Michael, but that's always a platonic kiss), even when Brian and Justin are separated. If that doesn't confirm Brian's love for Justin then I don't know what does!