I posted this to Kaz privately a bit ago, but I will post it for everyone since he isn't the only one who seems to feel this way...
Poppy is the most innocent of the girls that work at UC8. In the MC's mind, he will begin to care for her and look at her similar to the way he does Kinley. That she must be kept safe and protected.
Although Poppy is an adult, she is still 16 years younger than the MC and is very innocent and gets embarased easily. She is not sexually experianced and is barely even sexually awakened.
Her boyfriend, Glenn, is the first boyfriend she's ever had. The entire purpose of Glenn's exsistance is to make the player hate him and wish he was dead. It's a tension element.
As the MC, you will get the chance to guide and mentor her away from her jerk of a boyfriend and help her to begin to see herself as worthy of love and of being treated as such.
You will get to spend time with her and love her as a person and a friend, but she is too special.
Loving Poppy is like loving the sunset... It's beauty and wonder all rolled together. It can make you smile and cry at the same time. It's never quite what you expect it to be, but yet it is somehow always more than you expected. It's exceptionally personal and intimate, yet imposible to hold and keep. You could sooner own the sunset than to have Poppy as an LI.
The best thing about Poppy not being an LI though, is that she will have a very beautiful, meaningful, and memorable story no matter what other girl(s) you choose to pursue.
Hope this helps.