Your guy is a bit older, and in an open relationship with a man he's been with for a long time, and their relationship is fine, but there isn't a lot of passion compared to comfort. Our guys have been fooling around, and the boundaries are there to keep from catching feelings, but the lines are becoming blurry. It could also be a line where your guy succumbs to cheating after a long time of flirtation. Another option is that the three of them are in a poly relationship, and things start to become a bit uneven.