There is a lot that goes into whether one cheats or not, and whether their marriage will be happy or not. Tis pretty clear that infidelity makes marriages unhappy (feel free to google all you like on that one, there's literally thousands of research papers on the effects of infidelity on marriage- Tis among the worst things that can happen in a relationship, falling just behind losing a child in its effects). Therefore, if a successful marriage should be a person's primary goal when looking for a partner, then women should select men that are the least likely to engage in infidelity. Men with more sexual partners are both more likely to cheat and more likely to report dissatisfaction in marriage. Attractive men, men in high-ranking positions, ones that live in cities, and ones that cohabitate are more likely to engage in infidelity. Furthermore, high income couples are found to be more happy than those with low incomes. Therefore women should clearly select for men who have as few sexual partners as possible, are successful but not high ranking and live in outside of major cities, are average looking, and make more money than the female. Any guy with more than a few sexual partners, a position of power, not enough money, a guy that wants to cohabitate with her prior to marriage, etc etc should be removed from the running as husband material.
Seriously, you can't use one thing such as "number of partners prior to marriage" as an indicator of marital satisfaction. There's so much else that goes into it. Maybe the ones that had more partners got married later and had stressful careers, while the ones with only two or less partners got married earlier and were stay-at-home moms that had few other stressors in their life. Maybe there is a strong religious or social component that also affects happiness in the women with less sexual partners. Maybe the majority of the lower partner cohort was younger, on average, and thus had less time to reach a state of dissatisfaction. Maybe the group with a large number of partners was a smaller cohort, and thus could not be said to be representative of the general population, and maybe those that were in samples large enough to be statistically significant with less sexual partners had not yet reached a duration of marriage at which unhappiness results.
I'm very familiar with this study- there were only 418 participants that got married throughout the entire study, and the average duration of marriage was not that long at its conclusion, as the study was only conducted for five years. The oldest marriages studied are likely only 4 years in duration, and most of them significantly less. The ones that are longer in duration are more likely to be the ones in which the woman is older (and likely probably has more sexual partners), as women are more likely to marry in their late 20s and early 30s than in their late teens and early 20s. Thus these relationships are probably longer in duration than those of the ones with fewer partners, and less likely to be happy, as Tis time that wears on a marriage more often than what past you bring with you.
http://www.huffingtonpost.com/2014/08/21/more-sexual-partners-unhappy-marriage_n_5698440.html
http://www.guttmacher.org/pubs/journals/3214800.html
http://nationalmarriageproject.org/reports/
