So, why is that? Because, as a guy, I would straight-up take a grizzly bear to dinner if it showed the slightest hint of interest in me. But that doesn't explain everything.
This study just exemplifies the classic idea of supply and demand, as Discover Magazine explained in a separate article, "...when there are extra women, men face a favorable market and can behave promiscuously, offering little parental investment to their mates. However, when women are in short supply, they can demand relationship faithfulness and commitment."
The results of the Glasgow study suggest it's a numbers game; when there isn't any competition around, women also feel less inclined to engage in risky sexual behaviors with men. Because, why bother if you're not trying to impress your hot friend Cindy (that's a joke, BTW)? Fewer women, fewer hot-and-bothered men, fewer people to impress, fewer hookups. Sorry, Cindy's friend. And Cindy.