User blog comment:B214/Question and Answers 6/@comment-4547267-20160717213526/@comment-9595671-20160719151820

It's healthy and natural, mainly because for most of the girls, Issei did something that either helped them overcome their past which allowed them to start moving forward, and for the rest of the girls he did something others didn't, couldn't or wouldn't do. When someone helps heal pain in one's heart or is willing to go so far to help them, it's not strange for a strong bond to form, even become love.

To give an example with Rias, Issei went as far as to give up his arm to Ddraig just so he could beat Riser and free her from her engagement to him, and she NEVER asked him to do that. She even gave up the Rating Game so he wouldn't die despite it meant she'd have to be stuck in that engagement, and give up her dream of finding someone who loves her for who she is rather than as the heir to the Gremory household.

The moment Issei found out Rias surrendered from Grayfia, he refused to let Rias be submitted to that fate and crashed the engagement party. After he won, Rias finally realized the one she was looking for had been in front of her since that fateful day, and she fell in love with Issei as a result. They didn't force themselves to love Issei, they all chose to love him, that's the difference between their love being unnatural and natural.