Because research has been done showing a correlation between a woman's partner count and her instability in marriage. More partners tends to correlate with greater chance of divorce down the line.
source?
This study was quoted by media when it came out in 2014.
Before "I Do": What do premarital experiences have to do with marital quality among today's adults? (PDF)
by Galena K. Rhoades and Scott M. Stanley
The National Marriage Project at the University of Virginia
Description:
To answer these questions, we analyzed new data from the Relationship Development Study. Between 2007 and 2008, more than one thousand Americans who were unmarried but in a relationship, and between age 18 and 34, were recruited into the study.
Over the course of the next five years, 418 of those individuals got married. We looked closely at those 418 new marriages. We examined the history of the spouses’ relationship, looked at their prior romantic experiences, and asked them about the quality of their marriages.
Excerpt:
Prior research suggests that if the only person you had sex with prior to marriage was your eventual spouse, your odds of marital stability are good. Data collected in the 1990s on women who married a generation ago (men weren’t included) showed that women who had sex with only their future spouse before marriage were at no greater risk for divorce than those who waited until marriage to have sex (Teachman, 2003).
But that’s not most people. In our sample, only 23 percent of the individuals who got married over the course of the study had had sex solely with the person they married. That minority of men and women reported higher marital quality than those who had had sex with other partners prior to marriage. We further found that the more sexual partners a woman had had before marriage, the less happy she reported her marriage to be. This association was not statistically significant for men.
. . . One reason that more experience could lead to lower marital quality is that more experience may increase one’s awareness of alternative partners. A strong sense of alternatives is believed to make it harder to maintain commitment to, and satisfaction with, what one already has (Rusbult & Buunk, 1993; Thibaut & Kelley, 1959).
People who have had many relationships prior to their current one can compare a present partner to their prior partners in many areas—like conflict management, dating style, physical attractiveness, sexual skills, communication ability, and so on. Marriage involves leaving behind other options, which may be harder to do with a lot of experience.
Having had more relationship experiences prior to marriage also means more experience breaking up. A history of multiple breakups may make people take a more jaundiced view of love and relationships.