I mentioned this on the talk page and didn't get a response, I don't agree with the argument against him. Offscreen Villainy doesn't count in a character's favour, so why should an offscreen redeeming quality (especially one that's never actually confirmed) count against them?
Topic on User talk:DocColress
It's because it's not offscreen - Sloan and Bree are together for virtually all of their screen time, aiding each other in their acts of villainy, and there's never any explicit moment where they're shown to not care about each other or Sloan is shown to not love his wife.
Contrast this with another Evil Poacher, Percival Mcleach. His pet lizard Joanna is working with him for virtually all of Mcleach's scenes, but we see Mcleach treating Joanna like shit the whole time. He very explicitly talks to her like she's an abused girlfriend or personal lapdog for him rather than a beloved companion. We don't get any such thing in Sloan and Bree's dynamic, and as a result not only is the possibility of them being Happily Married real, but neither one stands out all that much from the other.