Mulitiple partners: macho man vs. easy woman? I sometimes hear from my friends that a man who has many female sexual partners is regarded as macho. But on the other hand, a woman with a lot of one-night stands is thought to be easy and desperate (without even mentioning harsher words).

So what's the reason for this? What are the psychological roots? I would say two most important factors are biological and social but how do they affect our opinions on this matter?
There are two very broad domains of theory that commonly address the double standard regarding promiscuity. Haven't got the time to lay it all out at the moment (would be happy to edit later to elaborate if you like; just comment on what aspects you'd like expanded!), but as you suspected, both biological and social factors pertain in ways that are essential to consider. In some senses the theories that focus on each oppose each other, as is a common consequence of any myopic discussion of an issue this complex and multidetermined. One answer focuses on one aspect, another answer focuses on another aspect, conflicting explanations and predictions arise, and hopefully the debate incorporates this...but unfortunately often, the original theorists who focus on one (their) side of the issue become invested and entrenched in their perspective, and dismissive of others (another example with which I'm personally familiar arose as the person-situation debate). Keep this in mind if you choose to read further into the literature on these theories. The best answer is often the one that is informed by both, or more accurately, all sides.

evolutionary psychology is the side that argues for explanations in terms of biological factors. Evolutionary theories pertaining to polygny and monogamy are not hard to find; even from a male perspective, there are competing motives to address here. Meanwhile, social-psychology has a lot to say about the influence of culture on social norms regarding promiscuity, particularly in women's studies. Again, a quick Google search on "women's studies sexual double standard" will give you more than you have time to read about that perspective. Bear in mind, these two are not the only ways to address the issue; they're just the most popular and most inclined to address this issue as a focal topic.

Enjoy, stay open-minded, and "trust no one [completely]!" :)