Sex can be difficult to sustain in relationships. While there are some who are able to do it, there are generally factors that can counter sexual excitement in relationships. For example, part of what makes sex exciting is risk and unknown. Think of the difference between the first time you had sex with someone and the 50th. Repetition and familiarity with a long-term partner removes the element of risk and unknown, which can also remove some of the excitement of sex.
Also, shame becomes an issue in relationships, which can inhibit sex. The more partners become known to each other as people with vulnerabilities, flaws, etc., the more shame increases. The fear of rejection, judgment, and ego annihilation increases, and can therefore shut down uninhibited sex.