I'm confused now.

Is it Hayden that is mumbling about a timer as he walks away or is it Brett mumbling while looking at Hayden? The bubbles have no arrows and it changes interpretation quite a bit.

If it was Brett doing the mumbling it implies that Brett was thinking of his arrangement with Hayden as a temporary thing all along. Like with football he was indulging himself with his desires before settling down and doing what his family expects of him. He could view his relationship with Hayden the same way. I think that fits the pause when Hayden offers to drag him to the gym and how much he laughed at using the safe word "honey". It would set the two up for more debate about the nature and value of relationships going forward.

But no one else seemed to interpret it that way. If Hayden was the one mumbling that leaves other possibilities. It's tempting to think that Brett has deeper feelings for Hayden than he's shown - because he pursued Hayden and because he was open to a real date. Also because he looks happy when Hayden gets closer to him. And because it would be awesome. But that's far from proven in the story so far, I think.