Speaking to Digital Spy, Brennan admitted that he was "actually shocked" by the reaction to the couple's recent kiss.
"I think we [writers] forget because we're always just so hard at work on the show how big of an impact it has," he said. "And then moments like that are a reminder of it.
"It's interesting, 20 years ago that would have been a huge, huge thing and now it's huge for a different reason. It's sort of exultant, where people were waiting for it and waiting and waiting and then finally it happened, rather than it being some sort of controversy. That's just a good sign for me about how this generation, the youngest two generations, are just different and have moved past a lot of the taboos that are sort of waiting to die."
However, Brennan hinted that Kurt (Chris Colfer) and Blaine (Darren Criss) may face some problems in their relationship.
"Mostly the biggest problem is that they're going to be sort of separated," he said. "Kurt's sort of in this little world apart from Dalton Academy, and bringing him back to [McKinley High,] a place that was actually really difficult, that he actually felt chased out of, and [them being] separated a little bit just by being at different schools... That will be really interesting to play out.
"But those two really like each other, I can tell you that."
Glee continues on Mondays at 9pm on E4 in the UK. The show returns with a new episode on April 19 at 8/7c on Fox in the US.
> Click here to read our interview with Ian Brennan