It is naturally odorless and has the purpose of keeping the vagina clean and healthy. It fights off abnormal bacteria and supplies the perfect lubrication during sex. If we had no vaginal discharge, the vagina would feel dry and itchy (which is common in menopause or any times of hormone imbalance). Sex would be impossible or extremely painful because the vagina would not be able to stretch.
When vaginal discharge becomes odorous and itchy, it is usually caused by an imbalance of the vaginal pH. This change suppresses the healthy bacteria, called Lactobacillus, and it is unable to do its job of keeping the vagina clean and healthy. This imbalance causes an overgrowth of a variety of microorganisms such as yeast, bacteria, and fungi - producing yeast and bacterial infections (BV). With these come odor and itching or burning.
Why does this happen? Many of us feel we did something wrong or we are dirty, so cleaning more will help. In fact, that will just make it worse - continuing the cycle of it feeling better for a day and then recurring, sometimes coming back even worse. Most of the time, excessive washing and douching is the cause of reoccurring infections and odors. Wearing tight or non-absorbent panties can also prevent healing.
Vaginal odor may also worsen after sex, due to the mixing of semen, sweat, and vaginal fluids. Semen has a neutral pH of 7.4, which is the same as period blood. But the vagina has to maintain a lower pH than that of the rest of our body - somewhere around 7.2 - in order to kill the abnormal bacteria that would cause infections and odor. The normal/healthy' Lactobacillus vaginal bacteria, like a lower pH. When semen is deposited into the vagina, the vaginal flora can be disrupted as the pH rises. It is this rise of pH that allows the yeast and abnormal bacteria to grow and overpower the Lactobacillus. Then, abnormal bacteria and yeast produce odor and irritation.
So make it a habit to jump out of bed after sex and rinse the semen out of the vagina with plain water only. Do not ever let soap get into your vagina, as it is very alkaline and can bring on recurrent infections and odor. Soap should only be used to wash your skin and the outer part of the vagina - the labia, and the anal area.