It's a matter of taste. Some people find the harem route unfun since it requires that all the girls be the sort who would accept a harem (limiting their personalty options), or winds up overwriting their expected preferences to make a harem happen anyway (undercutting our emotional connection to them).
Moreover, turning down sexual offers isn't necessarily unsatisfying, if doing so leads to a sex offer you really want; it might make that final offer feel more important and rare. Plus, it can add value to repeated playthroughs when different choices show different scenes.
I also think you overstate the problem. Just because the MC has the option to turn down sex, that doesn't mean he will instantly forfeit all other girls if he chooses to have sex. The game already spends a lot of time in the hazy divide that separates "dating casually" and "committed relationship." It's also possible for a relationship to hit a temporary snag (it's not often talked about, but something like this happened with Maya and Josy in Episode 4). Add in dealing with the consequences of infidelity, drunken experimentation, and girls up for open relationships, and I think there is plenty of room for sexy-fun-times without going full harem.
Lastly, keep in mind that turning down sex can still lead to scenes that are funny and/or sexy; consider losing the CUM-petition or wearing the dildo helmet when spending the night with Jill.