If the question was directed at me and I found later that I was different to what I was before, it wouldn't matter to me because my decision to stay by someone's side forever is not truly dependent on physical intimacy. It's much more based on how comfortable I feel expressing myself around them and finding that harmony between us, and usually a good romantic relationship is just an extremely comfortable platonic relationship that's interlaced with lots of intimate moments, whether it be through hugs, cuddles, or just that safe touch.
I think sexuality gradually stops mattering as you get older. But of course people are different and everyone has certain needs, if the importance of that aspect outweighed their previous decision to be committed, I wouldn't blame them if they chose otherwise, although a proper discussion can be had between the couple to reconcile the difference.