Well it's a game with multiple paths and endings. They were presented like that, as depending on the path the player is on, they are certainly all viable options - it's not really about screwing someone over, however none of them are the "ultimate ending" the one the couple set out to achieve. I mean if the player is on the cheating path, it's not a hard stretch to guess that Tony may be left for another bloke or he may play second fiddle to that bloke. Yeah Tony may be getting screwed over here, but Anne has been doing that for the entire game.
The speration path to me shows this option to be viable. The game warns you it could lead to a game over. And this was because of anne sleeping with Ryan after he was put on the no go list... We are yet to see the fallout of Anne's fumble with Martin, who is also on the no go list and a guy who cost Tony's last relationship.
Now I'm not denying there could be other endings. As I said, we are yet to explore the Cuckueen path, the swingers path is not really explored yet, Anne finding out about Tony's cheating is another bomb ready to go off.
And this would be the best ending, and by no means off the table, it's also the ending I want. It's the ending the game set out to provide way way back at the start.
But look at some of the paths we have now
Both Loyal
Swinger
Seperation
One cheats
Both cheats
All roads can't lead to the same place here. There will be endings that are not considered a win