I remember feeling that way before. I was with my first partner for a really long time (years) and thought at one point we would get married. I always wondered what it would be like with someone else. Damn, am I glad I left them and had sex with other people! The grass was definitely greener.
This is exactly why I won't date a virgin. Not that I'd think less of their choice, but sex is a very important part of a relationship and I'm not a person who can handle a poly relationship. It's totally natural and almost expected for people to question life-long monogamy with their first partner.