Hey there!
I totally understand what you mean and this topic is certainly one that gets brought up a few times. While I personally would not have anything against the girls having fun with each other ( as I showed in a few threesomes already ) I also know that a lot of people don't like this. They want the harem to themself and to themself alone - which is also totally fine, Imo.
There will be more threesomes like this in the future, though. They will be optional so if players dont want to see it ( like the threesome at the end of 0.18, for example ) then they can skip it.