Okay youre not being a jerk so Ill go over it with you.

Saying you have a preference is not the same as saying you will date anyone who is that preference or you will only date people who fit that preference. It is saying you find a certain something attractive, thats it.

Going from there when you first meet someone they have no emtional connection to you that would bias them in your favour, nothing. They have no fond memories you just met. Because of this they will look at the necessary physical attraction to determine if its worth trying something out with a person.

That does not mean that if my fiancee lost weight I would leave her nor does it mean that I am only with her because she isnt skinny. It means I like that about her because its a body type I find attractive.