Haselton et al. (2005) wrote:

A truism in psychology is that the best predictor of future behavior is past behavior. This is no less true in the realm of sexual behavior. Indeed, one of the strongest predictors of marital infidelity is one’s number of prior sex partners

(excerpt).

Kinsey (1953) assessed that women with premarital experiences were far likelier to engage in marital infidelity (excerpt), which has been borne out in several studies since then, with a greater number of partners corresponding with a higher chance of infidelity. Essock-Vitale and McGuire (1985) found that women who reported having sex with other men while married had significantly more sexual partners (24.5 vs. 3.9) than women who reported no extramarital affairs (excerpt). Whisman and Snyder (2007) surveyed nearly five thousand married women and determined that the probability of sexual infidelity increases with a higher number of lifetime sexual partners, with as high as a 13% increase in the annual prevalence of infidelity per additional lifetime sexual partner (excerpt). However, Treas and Giesen (2000) estimated only a 1% increase in the net odds of infidelity for each additional sex partner between the ages of 18 and first marital union (excerpt). In their study of how premarital sexual behaviors affect post-marital adjustment, Athanasiou and Sarkin (1974) found that respondents who reported extensive premarital sexual experience generally reported extensive extramarital activity, with the number of premarital partners showing positive associations with the number of extramarital partners, the desire to engage in mate-swapping activities, and lower marital satisfaction (excerpt). Forste and Tanfer (1996) examined sexual exclusivity among dating, cohabiting, and married women, and found that unmarried women with 4+ partners were 8.5 times more likely to have a secondary sex partner than a woman with no previous sex partners (excerpt).

One’s number of lifetime sexual partners wasn’t just highly correlated with marital infidelity but with infidelity outside of marriage as well. Feldman and Cauffman (1999) found in their study of adolescents that sexual permissiveness promotes sexual activity with a larger number of partners, which, in turn, increases the chance that sexual betrayal will occur (excerpt). In their study of infidelity in heterosexual dating couples, Barta and Kiene (2005) found that individuals reporting a past history of infidelity tended that have a greater number of sexual partners than those without a history of infidelity (excerpt). Maddox-Shaw et al. (2013) affirmed that the number of prior sex partners predicted future extradyadic sexual activity, or sex with others while in a relationship, in unmarried heterosexual couples (excerpt). Hughes and Gallup (2003) found that promiscuity (measured in number of sexual partners) is a good predictor of infidelity in women, with promiscuity among females accounting for almost twice as much variance in infidelity (r^2 = .45) as it did for males (r^2 = .25) (excerpt).

Pinto and Arantes (2017) found that sexual promiscuity doesn’t just have a high correlation with sexual infidelity (r = .595), but that it also has a high correlation with emotional infidelity (r = .676)(excerpt). In their study of female twin pairs, Cherkas et al. (2004) affirmed the high correlation between promiscuity and infidelity but also discovered that the genetic correlation between the two traits was .47, so nearly half the genes impacting infidelity also affect number of sexual partners (excerpt). Fincham and May (2017) listed a greater number of sexual partners in their list of demographic factors found to facilitate infidelity, writing that permissive attitudes toward sex, a greater willingness to have casual sex and to engage in sex without closeness, commitment or love (i.e., a more unrestricted sociosexual orientation) are also reliably related to infidelity (excerpt).

The following are from non peer-reviewed sources.

Running values from the General Social Survey, McQuivey (2019) found that people who reported four or fewer lifetime sexual partners, the rate of infidelity in the current marriage dropped to 11%, while for those who had five or more sexual partners the number was nearly double (21%) (screenshot). Relationship consultant, author, PhD, licensed marriage and family therapist, Dr. Athena Staik (2019) placed a “history of promiscuity” as number two in her list of “10 Predictors of Infidelity and Gender Differences” on Psych Central (popular news site for mental health professionals), writing:

Contrary to the myth, partners who’ve had many partners have a harder, not easier, time remaining monogamous. They are significantly more at risk of straying than those with little or no prior sexual experience

(excerpt).