No, solution is not to cheat. There is NO good solution here. They (Cooper and Billie) got themselves into the hole there is no good exit from. Specifically, with two kids now. They are sexually incompatible and the problem is exacerbated by the fact she has this sexual history that is so magnificent and string Cooper simply cannot compete with. Even if Brad completely despaired I am sure she would find someone else to satisfy her sexual desire. I honestly cannot blame her for this.SO, the solution is to CHEAT on her husband, take away HIS choices (since I presume he doesn't know) and SHE gets her cake and eats it too? This is the "best" solution -- for HER. NOT for her H, NOT for her kids, etc..
The tragedy of this situation and I think it happens in real life too often that young people being in love in general dismiss their incompatibility thinking it will resolve itself or no big deal. We have friends how had been together for almost 10 years. They had everything they can dream of: a house, money, a kid, parents support, etc. But they were sexually incompatible and eventually this issue crept into their relationship and marriage and destroyed it.
This is a show, obviously fictional but it describes real life dilemma with some fantasy (both men chasing her and very wealthy).This is the problem I have with these shows -- they romanticize cheating. How about they focus on the H finding out, divorcing her, the crap the kids have to go through due to HER cheating, etc? Why, because the reality of infidelity and the damage it does to everyone involved isn't "romantic" or "sexy".
BTW, H did find out about her cheating. But H did not leave her because he loves he and has this simply unrealistic dreams that she will be able to forget her sexual attachment to Brad. It will never happen.