I tried keeping a "friendship" with a gal (many years ago) after meeting and marrying my current GF
(wife). It was ok at first. Even my current went along with it, it seemed. The other gal met someone else, got married, ect. We all got together a couple times for dinner. On the outside it seemed great. But I finally decided it wasn't healthy for my marriage, as I saw(and my wife saw) I still had some feelings for the ex-GF. I cut off all contact with her, and haven't looked back since.