I personally find the female body beautiful, yet I am not bi-sexual.. I've had many a woman (when I was younger) approach that but whilst I admired their minds, their softness, emotions and bodies, attire it just wasn't there for me. I loved acting, singing so I guess I am somewhat also artistic to a degree... So I understand where your mind is coming from.
There is nothing wrong with sexuality and admiring it. In your position of sketching art it makes perfect sense to me that you are going to admire what it is that facinates you, excites you as an artist.. You are looking at things from an artistic perspective.
The only other thing out of the ordinary is that you compare sizes.. That could be you feeling in-adequate for no apparent reason, feeling that what you are looking at in its totality is better. Not a healthy way to look at things.
If you are picky and like things in proportion again, that's just your artistic side.
I wouldn't worry about it.