I believe that in most of the cases where ILYBNILWY is said, it's because the spouse has already developed feelings for another person. Otherwise how would they remember how the 'in love' feeling is like. Most of us have years long marriages, and that 'in love' 'infatuation' feeling has been long gone... yielding to a more familiar and less intense but stronger, unconditional love. When prompted, I would say in most normal relationships, passion can reignite when prompted...
I've been with XW for 23 years, and married for 14. Did I no longer feel the infatuation like I did when I first met her when I was 20? Yes. Did I choose to destroy the family and divorce her,? No.
The MLCer feels the infatuation and in love feeling with someone else, that's how they know they no longer feel it for you... I'm not saying MLCers don't have issues to resolve and inner child to feed, but they in fact do feel what they feel.
When they tell us how they feel, they are not lying. Not to say they are not liars, but when they say they are not 'in love' with you... that is how they feel. Just pointing that out, sometimes LBSers go into denial saying that no... MLCer is in love with me, they are just sick and lying to me... No, they don't feel in love with you.... maybe that will change maybe not, but don't kid yourself.
I know that to be a fact with my xW. She was physically repulsed by me. It may be all in her head... but that's all that matters.
xW told me her feelings are gone, and they are not coming back... She wanted to feel for me, and wanted to be in love, at first she said it'll takes a little time... but then decided that it can never come back. To me, love is a choice, if I will myself to love someone, especially someone I care about, I can... To her, it's an elusive thing, well beyond her control.
How scary is that? I have no control over whether you love someone or not... Say she finds a new husband, what's to say she won't lose the feeling in two months... How about my poor kids, what if she loses 'feelings' for them and can no longer love them. If everyone thought that way, what kind of society would we have?