Okay, to touch on that whole Magus/Schala incest thing, as a brother who has gone through his own deal of hell in order to protect his little sister because he loves her more than any other person in the world is not because of lust, but because of a deep bond between siblings. Siblings who share tragedy end up having really deep bonds, and so I really don't see any kind of sexual relationship between the two characters, unless you consider them to be step-siblings which I've heard is how they may have been originally written (not an expert so probably wrong on this but I recall reading that somewhere).
Now, if you consider a date hanging out with someone for an evening, I'd probably pick Frog as he's the character I think I would have the best discussion with. Robo would also be fun company.
As far as a romantic evening that may lead to a love interest? I'd probably say Schala because she falls into the "sweet and gentle" type I tend to be a fan of and she seems nice personality wise.
If it's straight sex, you gotta pick Ayla. Crazy caveman sex? Hell yeah.