There really isn't a taboo these days, IMO. Not in larger society, though perhaps in certain familial situations. I was the first to divorce in my family, followed by my brother and, most recently, my younger daughter. It happens. And the reasons are, at the end of the day, no one's business but yours and your wife's.
There isn't. Courts make custody determinations based on the best interests of the child at the time the determination is made, not based on some agreement made who knows how many months/years in the past by people who, at the time the agreement was made, weren't even planning on having kids.