You know, from reading everyone's responses, I get the feeling we are missing something important here. Mball keeps saying it is a reasonable expectation to have sex in a marriage. Others are saying it is too hard to determine who has the right to make that decision so have dropped the expectation part completely. I agree that reasonable expectation is really hard to work out. Mball says between couples, but then which one in the marriage gets to decide what is reasonable. I see this as rift with problems if the two people trying to determine reasonable expectations are miles apart on the issue.
The survey of the reasons for divorce are very interesting. It lists sexual problems at the bottom. So it would seem, as others have already mentioned, that stress from other life events lead in the end to sexual problems.
If a person is depressed, sick, or has an almost non existent libido, they obviously will not feel like having sex. For awhile the partner can understand this, but when it goes on for any length of time the partner usually starts to question the abstinence and resent it. How they handle it at that point can imo determine how their sex life will go in the future. If the person that desires sex respects the wishes of the other person who does not, not guilting the person or pestering, there is a good chance that the person will eventually come to the place where sexual relations are resumed. Mostly because they feel respected in a very important area of their lives.
However, if they are pestered or guilted in any way, they will likely be even further turned off by their partner. This can reach a place where they really no longer desire their partner and don't feel respected. When this happens, if they give in to the guilt trips or pestering, they will feel resentful. The more they do give in, the more resentful they become. Eventually, it will lead to the death of all sexual feelings towards that person. When that happens this usually is a crossroads in a marriage. There are two ways to go usually, one is accepting a sexless relationship that may be good in other ways, or getting a divorce. If the one person who wants sex cannot live without it, then I would say divorce is their only option or an open relationship where they can have sex with other people.
The thing is most of these things escalate to that extent because of expectations in the first place. Because we look for others to "complete" us we expect them to do things we want them to do. When they fail to do that, we get angry or resentful. No one is there to serve us. They are there to share their journey with us in the way "they" choose to. Just as we have that right also.
So personally, if I was in a relationship that was making me resentful of my partner in any way, I would have to give a great deal of thought to why I was staying. It is very likely if I'm not enjoying the relationship, neither is my partner so staying to gether for the sake of propriety isn't doing either one of us any good. Sometimes it's a fact that people are not compatible and it's okay to say goodbye. Sometimes the marriage has got to the state where you're partner just isn't interested in you anymore, and as hard as that may be to take, it needs to be addressed. I don't think it is ever right to expect the other person to serve us sexually. We don't expect it from people we aren't married to so why should we suddenly not respect the wishes of someone that means a lot to us. Marriage should not be an institution that takes the rights away from someone. That's my opinion obviously and won't be others.