A healthy sex life truly is the "life blood" of a healthy marriage.
Sex is the only thing which distinguishes marriage from a friendship or business relationship.
Sex is both a necessary ingredient to a marriage as well as a baromoter of its health, because a marriage which lacks a healthy sexual component ceases to be a marriage and, without fixing, will wither.
There are four reasons why your sex life may be suffering:
Orthopedic problems such as knee, hip, spine and neck pain can make sex very painful for one or both partners.
Alcohol, smoking and drug use can also affect the libido as well as proper functioning of the sexual organs.
Exhaustion can make it difficult to "get in the mood" so if this is happening then make a point for both of you to get more sleep or find a time and day where you're likely to have enough energy.
If you or your partner are dealing with any of these issues you've got to see a doctor (maybe more than one) and get it sorted out.
Let me tell you first-hand: my back was broken in three places and the pain kept from doing just about anything, which made me miserable and destroyed every relationship I was in. Only after I really took ownership and threw myself into getting better was I able to make a good love last.
Depression and anxiety can reduce interest in sex as well as preventing your sex organs from working they way they should (and need to).
Insecurity and negative self-image can also get in the way of enjoying sex with your partner.
If you're suffering with depression and anxiety then treat them just as seriously as a physical condition because they will take an unremitting toll on your marriage and, if you have children, your family.
Ironically, if you are (or wind up) taking medication to help treat depression or anxiety be aware that some of them can interefre with your normal libido and sexual functioning.
To handle insecurity and self-image issues (so long as they're not the result of belittling by your partner) you should work with a good coach or therapist. If your partner is emotionally abusing you then you should get the help of a professional therapist and, if possible, physically distance yourself from your spouse until the issue can be resolved one way or another.
Attraction between the "masculine" and "feminine" is what "turns us on" and makes us want to enjoy sex with our partner.
Masculine energy is about achievement, building and structure. Feminine energy is about connection, creating life and playfulness.
When the polarity between you and your partner is weakened the attraction between the two of you will also be weakened and your sex life will suffer, so you need to find ways to amplify your polarity at home.
This is even true in my own marriage. My wife teaches science (masculine because of structure) and I coach (feminine because of nurturing); when we forget to "balance" ourselves out our polarity goes down, we lose that *need* to be with each other, and our sex life suffers.
Many of the clients I speak with who have a polarity issue have gotten there because of work and/or role issues. In fact, these days the most common thing I come across is a women who is very successful at work and brings that go-getting/dominating energy home with her, and men who are so "enlightened" that they lose the rugged masculinity which their woman needs to be attracted to them.
The fix is easy: simple make a point to balance yourselves out and do things which amplify your natural polarity: sensual, nuruturing, and self-indulgent things (like bubble baths, massages, etc) to amplify the feminine and rugged, action-oriented things (like martial arts, building someting, etc.) to amplify the masculine.
Emotional intimacy and trust are crucial to a healthy sex life.
In fact, emotional intimacy and trust are a prerequisite for most women before they can imagine having sex.
When a woman feels she's been betrayed in some way, or is afraid that she might be hurt, her body (on a physical and neurological level) "shuts down" for sex.
When a man feels he can't share something with his partner he'll pull away and find someone he can open up to.
Trust and emotional intimacy are absolutely essential.