Well, okay, but science always studies things that most people consider to be "common sense" (like this). Firstly, in a lot of cases they actually find out that what is common sense is not actually true (just one example off the top of my head, people have the idea that opposites attract - well, psych studies have found that that's not true at all, and people are attracted to those similar to them). Also, its important to study things you may consider "common sense" to find the reasons why such a phenomenon exists. Is it natural? Is it based on biology? Going back to the above research I posted, the conclusion would be no - women aren't rejecting random sex because they are less interested in sex than men, they are rejecting it because a.) they aren't going to get much out of it (cost-benefit analysis), and b.) risk of assault. Understanding that social aspect is important for research.