The clip begins with Lee standing up and shouting into the night sky, "My name is Lee Adama and I love Kara Thrace." Kara is gigling on the ground.
Lee: "It's not like anyone will hear. We're in your cabin in the forest, right?" (Not sure what that is about, but they must have been having some discussion before Teh Sex.)
Lee: (Again shouting at the top of his lungs) "I love Kara thrace and I don't care who fraking knows"
Kara: "You're crazy. Get down here."
Lee: "I won't stop until you do it too."
Kara: (Still giggling) "I can't"
Lee: "OK, I'll do it again. "Lee Adama..."
Kara agrees to do it and jumps up, with her arms wrapped around herf because she's obviously quite cold. She starts saying "Kara", hesitates, then begins again and yells, "Kara Thrace loves Lee Adama."
She laughs some more then turns to Lee and they kiss. When they break the kiss she gets this serious, 'OMG what have I said' fearful look on her face.
We then cut to the boxing ring and Lee landing a hard blow that sends her to her knees. Both are badly beaten and bloody. Lee is saying something to Kara that we can't hear over the noise of the crowd. (I think I made out 'frak' though) and we see Helo telling her to get up. She them looks up at Lee and uses her leg to kick his legs out from under him. When he tries to get up, she kicks him in the face sending him spinning around and landing face down on the mat. I got the impression that from there we cut to his POV in the flashback since we had just seen evenst from her perspective.
Now, after seeing this clip I have to say I think it's Kara's "I love Lee Adama" that causes the rift and his downward spiral. He looks so fricken happy proclaiming his love for her and even happier hearing her say it. In his mind, this was it, they loved each other and were going to be together. But to have her tell him that, then disappear, and find out she married Anders, yeah I get why he can't bring himself to have anything to do with her. And I think I get why she did what she did, but from his perspective I can understand the rage and the hurt. And while I don't see this as cheating on Dee per se since I think he believed that relationship was over, that he marries her on the rebound/to get even is the wrong thing to do for both of them. Though, given Dee's actions with Billy my heart doesn't bleed for her as much as it could have.
Oh, and I think we get a glimpse of Jamie/Lee's ass. ;-)