So. Buck never told Eddie about his hookup with Tommy. It's been over a year. Not important anymore.
Buck has a kid now and is worried about other things unrelated to dating or having one night stands.
Eddie spends a lot of time at his house with Chris to help with Theo. So this time the fight it's in Buck's new kitchen.
Somehow Eddie finds out. And he confronts Buck about sleeping with Tommy while he was in Texas. The discussion get heated. The kids are not there. They get loud about it until Buck says.
"Why do you even care so much?. I was lonely, I was drinking and he was there. That's it"
"So just because he was there you fucked him in my house?"
"Watch your language, please."
"Come on, Buck. Our kids are not home"
The our kids is in the air for a moment.
"Eddie, you were a renter"
Eddie doesn't understand what that has to do with anything.
"It doesn't matter. It was a mistake and you don't care about it"
"But you don't" The words are barely there.
And Eddie decides to show Buck how much he really cares.
They get married and moved in together in less than a month after that.