I'd though I'd list a few studies which, though flawed, still show a link between sexual partner count and infidelity.
Sexual Infidelity among Married and Cohabiting Americans. Author(s): Judith Treas and Deirdre Giesen, Journal of Marriage and the Family, Vol. 62, No. 1 (Feb., 2000), pp. 48-60.
This was one of the first big studies looking into the the phenomena and controlled for an extensive list of variables including education, sexual values, sexual interest, religious values, marital duration and cohabitation. The study involved 2598 women between the ages of 18-59.
Some of the statistically significant positive predictors of infidelity were:
Sexual interest, defined as how often the subject thought about sex, (1.30).
Non permissiveness, defined as how negatively infidelity was viewed, (0.53).
Number of partners, (1.01). (N.B partners= partners after the age of 18 and before first marriage.)
Living in the central city, (1.47).
Marital duration, (1.03).
Sharing a mate's social network, (0.74).
Each year of marriage increased the odds of infidelity by 3% whilst each additional sexual partner (after 18 years of age) increased the odds of infidelity by 1%.
This study also looked into attitudes and 99% of respondents expected sexual exclusivity. The polyamory crowd are freaks.
The study seems to be flawed in its methodology. The women were divided into two groups, those who were married once and those who were married more than once. The once married were allowed to answer sensitive questions anonymously whilst those who were married more than once were interviewed. Of those married more than once, the data excluded sexual relations that had happened after separation but before divorce and infidelity was only measured for the most current or recent marriage. In other words, if a woman had been unfaithful in her previous marriage it wasn't included. (I don't know who dreams up the methodologies of some of these studies, they almost seem to want to artificially lower the rates of infidelity)
Another study to look at infidelity, available online, is;
Cherkas et al. (2004). Genetic Influences on Female Infidelity and Number of Sexual Partners in Humans: A Linkage and Association Study on the Role of the Vasopressin Receptor Gene (AVPR1A). Twin Research, 7, 649–658.
This study was flawed in that it sent out a questionnaire to 3654 pairs of female twins aged between 19 and 83, of which only 46% bothered to complete and return it. Personally, I feel that this biases the study towards "conscientious" types, introducing another confounding variable into the analysis. However even with this bias toward the "goodie-goodies" a link between promiscuity and infidelity was statistically established. The mean number of lifetime partners was 4.67. The unfaithful had a mean of 7.73 whilst the faithful had a mean of 3.78. (p<0.001) all have their faults. But with regard to promiscuity, all studies show an increase in the risk of divorce and infidelity correlated with cumulative partner count. People may legitimately argue about the degree of the effect but no study shows a protective or indifferent effect.
A good review article, available on line, is;
Infidelity in Committed Relationships II: A Substantive Review. AJ Blow, K Hartnett, Journal of Marital and Family Therapy. April 2005, Vol. 31, No. 2, 217-233.
Perceptive readers will note that I have mentioned nearly all the major studies looking at promiscuity and infidelity. I have not been selective in the studies that I've presented. What's really interesting in how little research has actually been done on the matter despite such a strong correlation. Now, I'm fully aware that correlation is not causation, but the whole raison d'etre of statistics is to establish correlations which may be causally linked.