Sexual Relationships 
Sexual Health 
This can also apply to boyfriend/girlfriend relationships as well; I just didn't want the title to be too long. Well, the reason I am writing this is because I have seen a lot of questions having to do with sexual desire on this site and I was curious enough to research about it, and here are some possible reasons why a girl or guy's sexual desire is so low and how to fix it.
The basics
Let's start off with some estimated statistics and facts of common problems having to do with sexual desire in relationships.
-As many as 43 percent of women and 31 percent of men have a sexual dysfunction problem.
-According to a survey 20 percent of American couple's are having sex less than ten times a year.
-Low sex drive complaints are the number one thing asked about to a sex therapist.
-There are many things than can cause a low sex drive, it is not a simple fix.
So with looking at those few points alone, you can tell a low sex drive is very common in a lot of relationships and it is by no means a rare occurrence. But now let's get started with the possible reason's for a couple to have no sexual desire and the possible fixes.
Fix/reason: Medical issues
I will get through this one first because there are so many possible medical issues that I won't list them all. It could be a low libido, hormone deficiencies, anti-depressants or it could be other things. Usually when it isn't temporary or when you tried to fix it yourself and fail it's more than likely something medical. Many medication are out there to fix someone's low sex drive if it happens to be because of something physical or psychological.
Reason: Society
Have you ever been so stressed one day where you just didn't want to do anything and it effected many of your performances you usually do? Well stress from a job or somewhere in society can effect sexual desire. A high stress job or something that stresses someone out can indeed make someone not want to have sex. Let's say every day one of the married people come home stressed out, more than likely the last thing on their mind is sex.
Not only stress can be involved but being physically tired can also effect that. If someone works a ten hour shift, sometimes they will not want to have sex and if it's very common that someone does then well..you see where I'm going with it. Or if someone tries to have sex with a job on their mind they may not keep being erect and that may lead them to not try at all.
Fix: Encouragement and support
This leads off of the society point. To fix a lot of low sexual desire, it is very important to encourage them to keep trying without too much pressure will help. Supporting them as well and make sure they realize that you know what they are going through can help a lot too. Sometimes someone is just going through a rough point in their life and all they need is just a mutual understanding and some sympathy. Those can go a long way in a relationship.
Fix: Counseling
An alternative to medication. Sometimes it may be just a psychological problem and sometimes women/men do not like the fact that taking a pill or some sort of medication makes them turned on and not actually them. This is where counseling can help a lot, there are many counselors out there that can help these sort of problems in a relationship. It is important to realize that medication is not the right fix sometimes and counseling can go a long way.
Reason: Conflict and resentment
Another big reason is conflict and resentment. When conflict is brought up a lot and arguments happen more often than not, this often leads to a low sexual desire because of the build up of numbness and no sexual feelings. This is why often in bad marriages, sex happens less and less. But obviously not a lot of sex does not equal a bad marriage as shown in other examples.
Fix: Communication
First off I want to say no offense to anyone who asks these questions on this site. In my opinion I think there isn't much communication between couples that ask the questions about sexual desire on this site. But communication is so important when trying to fix these kinds of problems. Without proper communication, nothing would be fixed. Sometimes having a serious talk can solve a lot of issues. This can be done in a respectful manor and of course consideration for each person. This can also help you learn more about one another by expressing turn-on's and turn-off's.
Reason/fix: Body image
When someone doesn't perceive themselves as attractive because of maybe aging, gaining weight and even pregnancy. This is why communication is so important, to communicate to your love in your life about how much you like their appearance. That could change how they think of themselves dramatically.
Reason: Substance abuse
If your partner is an alcoholic or is doing drugs like cocaine or anything else that is highly addictive and damaging, that can effect their sex drive as well. It's important to help your partner seek help if this is the case.
Reason/fix: Long lasting relationships
Finally, I think a more common reason is that when a relationship starts going longer and longer into a few years, sex may get rather boring. Not to all couples but I am sure some face that sort of feeling. That's why it is always important to try new things with each other, always try your best in bed and to make both of you happy sexually. Speak with each other about fetish's and other sexual related topics to make things better in bed.
Hopefully this will help someone that reads this or at least gives someone an idea on why people may not be having sex a lot. These aren't all the reason's of course but....well I tried to keep this as short as possible. There are other reason's but I think these may be the main reason's.
Also another note, I am in no means a professional at this so please if I said something wrong or something that doesn't make sense please correct me so I know what I have done wrong. But I think most of what I have said is pretty straight forward and not much science behind it. Thank you :).
Most Helpful Guy