Er ... this is Asian dramaland, honey. Not Western TV. You're probably going to have a difficult time finding a drama like that.
Ok, perhaps the first question to ask is what do you define as a manwhore? Maybe you and I have different ideas.
What are those dramas with manwhores in them? In my experience with Asian dramas, it is quite taboo to portray both women AND men in such terms.
Sure, you can find dramas where the ML may have had or still have a wife or gf prior to or upon meeting the female lead, but he's not portrayed as actually being a manwhore. Usually the relationship with their wives/gfs are on verge of collapse so that he will be free to pursue a relationship with the FL. Other women, besides his current woman & the FL, are not usually involved.
You have cheating dramas - like Temptation, which I loathed - where the male lead cheated on his wife with another woman, but he was never cast in the light of being a manwhore. Supposedly, he simply fell out of love with his wife & in love with the other woman, the FL (he fell in love with her money, if you ask me). The execution of his journey was poorly done, which is why many audiences, perhaps most, hated his guts and hated this drama. But I don't think anyone would look at him and think manwhore right away. Golddigger, disloyal shameless opportunist, a kept man, perhaps ... but not a manwhore.
At least I wouldn't. When I think of a manwhore, I see a man who has multiple sexual partners, often at the same time, hopping from woman to woman, who deliberately lies and deceives, a lot of times with an abundance of charm, to maintain these multiple encounters. I've never seen a character like that in conservative Asian dramas.
The one drama I can think of that perhaps comes slightly close to the ML being a manwhore is Que Sera Sera.
Aside from that, I can't think of another one.
As for dramas where the female lead is a slut ... even rarer, probably none existent. I'd love to know what those dramas are too, if they exist, so I can avoid them.
One of the reasons why I appreciate Asian dramas is the value they put on having a relationship rather than just letting the characters f***k around with as many people as they can. Sure, there are one night stands, but the characters almost always end up having a relationship with each other. I'm tired of how *everything* in western TV has to be sooooo sexualized, even when it's not necessary.
Sometimes Asian dramas have a horrible tendency of portraying adult relationships as too childish and chaste (the polar opposite of its western counterpart), as well as infantilizing the FL ... so I get your frustrations, but sometimes they manage to get the balance right (where the characters behave like adults in an adult world), far more so than Western shows that tend to put too too much focus on sexaulizing everything & everyone, esp if we're thinking the typical Hollywood sponsored ones.