I think it's a combination of misconception for some, and for others an overabundance of raw sexuality that often overrides the social norms for some people when "on the hunt."
The misconceptions aren't totally invalid though. It's human nature to want to take what we don't fully understand and place it in a box and top it with a bow to make it that much easier to justify or rationalize the courses and actions we take when acting upon the misconception. Both individuals tethered together in a smaller box where assumptions are drawn; all the while residing within a larger box that's considered to be mainstream mindset and world view. 1 + 1 has to equel 2 right? Problem is it often doesn't. It doesn't mean that these assumptions are right, but looking at the big picture through such a fractured lense as this one can see the logic - flawed or not - with this scenario.
Then there are others who don't have a misconception at all and readily realize that people are individuals with different likes and dislikes, and firmly believe that the one that they are pursuing may be just as likely to spurn them based on their own interest and moral code regardless of any commonalities between the two . A commonality between 2 people linked together by little more than a sexual catagory or label does not a relationship make. Many of these people already know this, but that is not their point. Their point is a numbers game. Alamo effect if you will. Little chance of success with typical one on one propositions, but if you throw enough troops at the wall you'll overtake it eventually based on the numbers. Kind of like the old adage where if you ask 100 women to jump in the bushes with you and get it on like rabbits - 99 of them will probably slap you. For people like this it's all about the 1 that takes you up on it that justifies their course of action.