There a few problems I see with some of the arguments in this discussion. First and foremost is that hutts are hermaphrodites not asexual. For those who don't know this means that they posses both male and female genitalia, which is a characteristic they share with many gastropods. So as disgusting as it sounds, it means Jabba either was artificially inseminated or got knocked up by another hutt.
As far as Jabba's attraction to humanoid females, it is stated in several books(Han Solo Trilogy) that his harem of slave girls was considered a "perversion" by some other prominent hutts, who saw it as something similar to bestiality. So yes Jabba was "attracted" by Leia and Oola.
Now why did Oola resist? Well it wasn't because she was ditzy (where did you get that from?), I'm pretty sure of that. Oola had been Jabba's slave for quite sometime and in that period was subjected to Jabba's "perversion", something that was obviously very traumatic for her. She had resisted him several times before and had been punished with beating by gamorrean guards (Tales from Jabba's Palace). So when Jabba pulls on her chain, his intentions clear, she resists hoping to keep away and suffer a beating rather than another night with her master. Jabba decided he had had enough of her disobedience and pulls her over the trapdoor and drops her in.