The feeling of being obligated to have sex with someone you're just not sexually attracted to is very unpleasant. Perhaps, at first, you (in general) force yourself to do it on a regular basis, but after a while, it becomes more and more difficult. Problems like this have a way of reinforcing themselves and unless something drastic happens to turn it around, it just continues to get worse. After a long time of being in this situation, you might have gone from not be sexually attracted to the other person to actually being repulsed by them. It really is a lose-lose situation, as both sides of a relationship like this are getting a bad deal. It's not fair and IMO, it's better to part sooner than later.
This is different than a relationship where neither person is interested in having sex with the other, but they love each other and stay together for other reasons. It's when the feelings are not mutual that a problem occurs and I can't think of many ways to resolve that. I've said before, I am not able to talk myself into being attracted to someone...but I'm guessing others may have had luck with that.
Life is too short too spend it giving or receiving mercy f*^%@. Sorry to be blunt, but I'd rather be told the real deal, then to go through the heartache of repeated rejection when someone is not attracted to me.