For my sins, I am continuing to read a novel starring Mr. Perfect Super Guy, who spends his quite copious spare time bagging the hot babes who reliably throw themselves at him even though he's not very good-looking and really has nothing to offer. (Plot? There's supposed to be a plot?)
Anyway, trudging through this book has led me to think long and hard about how to create promiscuous characters who don't make the reader want to reach through the pages of the book and strangle them. Obviously, you might at times want to create a character who is both promiscuous and an asshole, but you really don't want to do it by accident. And it's a little tricky, because it's safe to say that the large majority of readers have at one point or another in their lives been romantically disappointed, so you want to avoid triggering the "THAT BASTARD IS JUST LIKE MY EX!!!!" reaction.
How do you that? Well, it helps to understand that the main pitfall of a casual sexual relationship is that sometimes people assume or hope that a casual relationship is merely the first step to something more serious. If your character is someone who has zero interest in having the relationship become more serious, and you want the reader to like that character, then your character needs to guard against creating expectations in the other person that will never, ever be met.
In other words, when interacting with their this-is-just-a-casual-thing buddy, you character should NOT:
1. Make plans for the future.
2. Take the other person to meet their parents, or agree to meet the other person's parents.
3. Accept lavish gifts.
4. Tell the other person at length what a great catch they are.
5. Agree to be the sole source of emotional support or companionship.
NO, a quick talk at the outset of the relationship establishing your character's expectation that this relationship will just be casual is NOT ENOUGH. "Let's keep it casual" is not a magic spell that guards against emotional attachments and romantic expectations for all time, especially if your characters start spending a lot of time together. (And WHY are they spending all this time together?) The other character knowing that your character has other lovers also does not magically fix everything--it just makes the other character look increasingly desperate as they take your character on lavish vacations, have your character meet their parents, and repeatedly grill your character for details on these other lovers. (Honestly--that way leads to bunny boiling, OK?)
A likeable character is 1. aware that they can harm others, and 2. attempts to not harm others. They don't necessarily succeed, but they have to do more in that regard than sit around and think about how goddamned lucky they are.