In a Harem themed erotic/adult VN/Game, I usually prefer it be a mix... Add in some corruption, add in some romance, add in some mutual lust, and so forth... And as far as one by one or all at once? One by one is usually the preference of most folks... Regardless if your the developer trying to underhandedly prolong development (shame on you if that applies to you), or a player/reader... If you don't have some varying foci, then there is a tendency for the product to end far too quickly, or become boring early... The top end VN/Games out there, that include both the harem option as well as corruption theme, tend to offer up various focuses to allow for different options... If it was all at once, options would seem less prevalent and there would be only one focus point...
Now do not get me wrong... I'm sure there are some folks who wouldn't mind all at once, if it was done right... It will always boil down to how the game/visual novel plays out... That could include things like story flow, plot/s, game play mechanics, and/or developer goal/s...
I'm surprised you didn't also offer the option of a few at a time...
Zip