In the Sopranos episode S05E12 titled "Long Term Parking" (at about 42 minutes), Christopher Moltisanti is apparently thinking about betraying his comrades and entering the Witness Protection Program, following Adriana's proposal. In the street, for some seconds, he looks at a young couple with kids. Does this influence his decision? How?
Though somewhat opinion based, I think it's relatively safe to assume that this seals Adriana's fate. Chris sees his future family life in the gas station: nagging kids, annoying wife, a beat up car and, most importantly, a very tired and fed-up looking husband. Chris's face says it all, this is not the future for him. His next move is to go to Tony, which leads to Adriana's demise. It was actually a slightly unsubtle way to eludicate his thought process.
You're talking about the episode "Long Term Parking". In that episode, as Christopher is contemplating turning informant, he goes to buy a pack of smokes and sees a family of schmucks piled into an old beat up car. Chris realizes that he doesn't want to be like them, working for a living and getting nowhere, so he decides against turning into a snitch.
Christopher knows that Adriana cannot have children. He's watching the scene thinking that this loser guy has a better life than him, that's why he turns to Tony.