It comes natural, and therein is the problem. Too much porn leads to unrealistic ideas as to what is in fact sexy and or what "turns you on". This is most likely why that one girl (a turn off to you) didn't get her bone. You literally condition your mind on a daily basis by all of the ambient stimulus around you. You make concious as well as subconcious evaluations and the brain stores all of this information for use when it is needed.
I get sick when I smell cabbage being cooked for instance. This isn't a voluntary reaction but an involuntary one, because I've conditioned my brain to trigger that sickness. How can anyone shovel nasty stinking manure everyday at work? They're conditioned. I bet you missed a good piece of ass because of your subconcious. You wanted that pussy and wanted to get off conciously but subconciously you hated the idea of a fat ugly girl. Let me be the first "old man" of maybe many more in your life to come to tell you that fat ugly girls are some of the best pieces of ass around. I won't go into details right now, but I think you get the idea. I do know of some exersizes you can do to gain more control over your subconcious and its fairly simple and basically just intentionally refocusing.
Our bodies adapt daily so you need to adapt yours to accept less than A1 Porn quality sex. Stop watching porn altogether. Concentrate on the big gals, everywhere you go, undress them with your eyes and imagine pounding them caveman style. Just raw sex, no relationship, just sex. Anything better will be an automatic so concentrate on the low end of the scale. There are no culls or rejections. Its all good. Focus hard and long no matter where you are, big gals are sexy and sex bombs. When you fantasize, make it a fat girl. Next, convince yourself that it is not their duty to be pretty and skinny for you, but rather your duty to rock their sexual worlds. You have to blow their minds, all they have to do is consent and be willing. Once you can be sexually aroused by them, everything else is cherry man.
Why did I pick fat girls for your focus? I didn't, you did, when you proclaimed your lack of attraction for them. Exercising and conditioning our mind is everybit as important as any muscle.
Please be kind and courteous as well as show respect for any woman anywhere no matter their beauty or lack of, at all times!!