The greatest challenge that confronts monogamous relationships today is to maintain the sexual passion once the newness of the initial months or years has worn off. You don't have to have a sexual problem for sexual desire and excitement to wane.
Excitement tends to wane for three reasons.
The stress of life in our culture today. Many of us are so busy with two careers, children, cooking, errands, etc. that we are left exhausted with little quality time left over for sexual or emotional intimacy. In the priorities of life, our sexual relationship gets relegated to last place. The second interfering factor is the suppression of feelings. Unfortunately, you can't suppress some feelings while leaving others free to grow and expand.