A person's reproductive goal, both in evolutionary times as well as by modern standards, is to raise successful offspring. Parental investment is a strong predictor of this.

Thus, a woman benefits by selecting a loyal man rather than a promiscuous one. For her to marry a loyal man with strong genetics (e.g. an attractive man with a limited sexual history, indicating a low disposition toward promiscuity) is optimal. A woman who disqualifies men because they've had a low number of sexual partners is never going to accomplish this.

Anyway, the obsession over the number of past sexual partners means nothing in an evolutionary context, since the whole concept is a social construct. Pre-numerate cave dwellers would have lost count pretty quickly.