The first two years of a relationship are usually considered to be the most exciting. After that, according to psychotherapist and author M. Gary Neuman, couples have to work to maintain that initial level of intimacy and excitement.

Neuman conducted a research experiment with 400 women who were either happily or unhappily married to find out how much sex happy couples should have every month.

Business Insider readers will a 20 percent discount on Neuman's products for a limited time by using the promo code "businessinsider." Click here to visit his website.