When I didn't have any film material yet and read the transcript of ep 202: "I wish I could forget".
But I tried once to spot the exact moment when Brian was lost and it went like this:
first stunned look when Brian spotted Justin under the lamppost - first kiss - "what is this, a missile launch?" then telling him about the most famous shower scene since psycho - look after "I don't believe in love" speech (Brian: I won't go down without a fight!) - happy grin while lifting Justin up in "let's hear it for the boys" scene - "I made it an early night" (reminiscing about second night with Justin) - and Brian was done.
Maybe the shotgun scene in ep. 117 was the first time there was only teh love and they were in the same place and again in eps. 202/203