Ren can't emotionally fulfill himself without ruining himself socially. At the same time he knows Hina is emotionally in a bad place and that Enabling her by using Change really bothers him and isn't good for Hina either. On the flipside, Ren is never going to look at Hina "that way". While she does gain some genuine fulfillment in "stretching her wings" as Ren at school and in sports, she essentially inherits his original problem: As Ren she would be a boy attracted to other boys and only faking interest in girls because that's what everyone else expects.
I'm just afraid this is going to end with Hina Changing with Ren-kun and jumping off a roof to "force" Ren to be "happy" by living as Hina and marrying Makoto or something.
(As she either doesn't know Ren has some hangups still, or understands Ren's aversions to being a girl are minor in comparison to what he gains emotionally from the situation)