Okay, first of all, back off Yuki. I agree, she's being pretty selfish, but fact is that this is her first relationship with a boy and she's probably a bit freaked out about it. My guess is, now that she has found a way she can actually appreciate boys and see a use for them, she wants to work with that as much as she can.
Besides, it's not her fault that Gary was born without the DNA that coded for certain parts of the skeletal structure. Girl's got to look out for herself. Frankly, this is just even more reason that Gary needs to change.
Second, it is possible that the reason she calls Gary the best lover ever is because she has feelings for him other places than her crotch. I'm fairly damn certain Gary's hypermonofocused talent shouldn't be able to out-do Zii's experience if it came down to sheer physical skill. That is, of course, a good development, she just hasn't realized it herself yet.