There is no universe in which a sexually explicit reverse harem needs to feature a freshly 17 year old girl. There was ZERO benefit added to the plot, and NOTHING would have changed if the setting had been a group of 20 year olds in college vs 17 year olds in high school. This is sick and pedophilic. 

The story was interesting, and I would have enjoyed continuing, but the only thing that made it palatable was mentally altering their ages. 

I will never recommend this series, and I refuse to support this author if they continue to write about literal minors having explicit sexual interactions.

I'm not a prudish person by any means, however if your story features minors, your intimacy should be closed door or fade to black. If you wish to include detailed descriptions, then simply age up your charaters appropriately.