I'm not as familiar with animie as a lot of members, so I hope that you don't mind if I use sci fi examples here.
Take the example of Jack O'Neill from Stargate: SG-1 and his ex-wife Sara. They both loved each other very much, and despite their marriage falling apart over the death of their son, they continue to care about each other, even if they will never get back together. This is canon. Jack pined over her for two seasons, said the thought of her had given him the strength to hike out of the Iraqi desert with nine broken bones, and keeps a picture of her in his house seven years later. The one time Sara was on screen, she obviously felt the same. Sara: We were pretty great together, weren't we? Jack: We were the greatest.
So Jack loved Sara very much in canon, but they aren't ever getting back together. Why can't people leave it at that? Why, for example, in Jack/Daniel stories (and don't get my wrong, I love me my Jack/Daniel) does Jack so often reflect that this is not only the best relationship of his life, but that things with Sara pretty much sucked? I mean what's the point? What purpose does it serve? To assure the reader that this is Tru Luv? I think that I can accept that Jack loves Daniel with all of his heart, while still having had another loving relationship in the past.
The same thing happens to Carolyn in The Sentinel. She and Jim couldn't stay married without killing each other, but now that they have more space they are friends again, and after one season she left the show anyway. There is no need for Jim to reflect, or worse still say, how horrible she was to live with and that that relationship had never mattered. The man seems to enjoy dating a lot of women; he's obviously bi rather than gay; it doesn't detract from his love for Blair to have had a meaningful relationship with a woman in the past. Really.
But worse than picking on the Ex-Wife is picking on the Dead Wife. It's awful enough that these women had to bite it to provide motivation for the male main character in the first place (I am sooooo sick of girlfriends in the refrigerator). Why does anyone feel the need to say that, to use an example that acaciaonnastik mentioned, Daniel only "settled" for Sha're (the wife that he loved heart and soul and mourned for deeply) because he couldn't have Jack. The poor woman can't even defend herself anymore. Let her rest in peace (or, better still, write an AU where she's alive and still kicking ass).