I figure the opposite is more common, but I'm definitely more interested in stories of those who have "settled down" rather than expanded.
I'll admit that the primary reason I preferred open relationships was not necessarily for my own gratification, but so I didn't have to feel solely responsible for my partner's gratification. I love the concept of the elimination of jealousy in a relationship, but I've learned that an open relationship isn't necessarily the antidote for that. I'm still not a jealous person at all and I'm never hurt by someone seeking gratification beyond me (strangely enough my experience is that it's usually the more promiscuous partner that's more prone to jealousy), but to me there's the stress factor of worrying about STDs and worrying about involving new people, when having a trust-based relationship with one person just sounds more relaxed to me as I age. I honestly have a mild panic attack every time I have sex with a new person, and I used to like that adrenaline, but I don't seek that rush anymore.
That and family is becoming more of a value to me. I'm losing a parent currently and the idea of having my own family as I lose the one I came from is starting to seem crucial for my future well-being. Randos come and go but having a committed family sounds very healthy and rational to me at this point.