Yep, they needed the artificial roadblock of 'I'm a guy' to progress through the harem/dating conceit of the manga. But the fact that the whole reason his family wants him to get married is to reproduce adds another twist to Kinosaki as a love interest.
also in the earlier chapters gero said that their bloodline will end with him cause he doesn't want him and his sister to get married is kinda a foreshadowing (I think) and I would really love it if he doesn't end up with anyone and will just be platonic with everyone including kinosaki
What if Kinosaki isn't actually a guy...
My theory as to why they gave him a male sidekick who looks like a girl is that if it was just a normal dude he'd be a rival and it was just a normal girl she'd be a love interest. This way, he's surrounded by only "women" but the one who's always there is not a potential love interest (at least so far), so the plot is able to advance. The same thing could probably be achieved if they made "her" a lesbian though