Monogamist (noun)

1. A person who practices or advocates for the practice of having only one sexual partner at a time.
2. A person who is in a romantic relationship with only one person at a time.

Origin:

From the word "mono-" meaning one and "gamos" meaning marriage.

Examples:

1. John is a monogamist and has been with his partner for over 10 years.
2. She believed in the traditional values of marriage and was a monogamist.
3. As a monogamist, he didn't believe in having multiple partners.
4. The study found that most people are not naturally monogamous.
5. He considered himself a monogamist, but he had trouble staying faithful to his partner.