Obviously, if you have the relevant harem, then those interactions obviously change in nature. But it is so easy to mess things up, that you're better off playing safe rather than being sorry. Most girls can be met when they are the only one in a specific location (Sasha in the morning in the bathroom, Sasha in the bar, Shiori in the strip club, Bree in the maid cafe, Samantha in the pub at midnight etc.) that it is more sensible to just find lonely girls who are in need of a kiss), rather than accidentally have witnesses to the more affectionate interactions.
Better safe than sorry.
In general, other than Kleio, it is easy to increase kink points for the girls in the game, so the rewards are quite negligible for most part. Conversation topics (love, sex, food, and television are very useful topics), spanking them, or picking the right options during events should mean that all the girls will have hit 50 kink by winter, if not sooner.