Guys, you're really going over the line here. We have speculated that this was a possibility since last season.
Stannis has always had this choice waiting for him, and because you thought he was an unbelievable badass, you're now rejecting it.
Stannis watched as his wife's family was burned alive, and we've had multiple characters on this show tremble in fear at the idea of him and his red priestess ruling.
He's been with this woman since day one and just because you thought he'd eventually reject her (as in a traditional plot), it's now bad writing?
Stannis would have killed Gendry, an innocent boy. There are at least two precedents for this behavior.
It may have not been the way you wanted it to turn out, but it's very much in keeping with what we know of Stannis. He's a dutiful leader who will do absolutely anything to rule.