It's an interesting cross-roads to get to. Brad and Robbie have been together "officially" since 1985. They got settled into an insta-family with Jeanine, Darius, JJ, and Will in 1986, by the time when they were just 23. Now it's fourteen years later....they're seeing one child going through his senior year of high school, which has to make them feel old. They're seeing the other two hitting their teenage years, and are probably going through what every parent goes through when they realize that their babies have been replaced by young adolescents. Now that the kids don't need them as much and they're beginning to edge towards an empty nest, it's time for them to start rediscovering what each one is like outside of their family. It makes a lot of sense that they're at this point- the kids are growing up, the 90's business boom is just about ending, and they got settled down when they were so young.
I do agree with the person upthread that Brad's kids would never forgive him, or take a long time to do it, if Brad wound up sleeping with Matt, who is for all intents and purposes their older brother.