Well unlike others I am actually glad it went in this direction. By all flags it would have changed Yurina here and Hina would end up with a reverse-harem. Now THAT is the "predictable, risk free option.".
But no, the manga is going in a different risky situation. Not just with Yurina, but by also throwing in that other cat-girl into the mix, who seems to be aiming for Yurina as well.
As for "consistency"? Friends, do you know about real world viruses? Not even they are consistent. There are people who are completely immune to even HIV, or are only mildly affected by the symptoms of infections. So why do you expect a fictional magical virus to be any different? Honestly, the "infection" angle is kinda brilliant, as that allows the author to do all kinds of different combinations.
On the side, I am amused. In the previous chapter majority was "Oh nooo, don't take away the boobs!" and here the first posters are "ugh, what an asspull." LOL