I think there are two issues here.
The first:
There are different definitions of racism being used here in the topic. Just as there are different definitions of the word "discriminate". Discriminate can mean the act of creating distinctions without merit, separate people based on arbitrary characteristics that don't do justice to what is being demanded of them. Or it may just mean "separate" , "distinguish". One term is broader and the other more specific and yet there is a profound relationship between both of them, but they have distinct meanings.
What is happening here is that many are using the term "racism" as "separate by race", but using the connotation of "disparage a particular group of individuals, treating them as less capable or qualified just because of their race". Except that even though both are valid meanings for the word to some extent , we need to be careful not to confuse them.
We can say, for example, that "black-skinned people are less susceptible to sunburn" - and this is a distinction by race. That, technically , is "racism" by that definition, but that is not the definition of racism that many people have of the word. The mere enumeration or distinction of a certain characteristic present in a given group that can be expressed by their race should not carry the negative connotation of the term racism, albeit it's still a differentiation by race.
The second:
People feel that they have the prerogative to "require" the acceptance of a certain standard of beauty from others, while ultimately beauty is a subjective and personal experience. That reminds me, for example , of the advertising of certain feminist groups who say that "everybody is beautiful". "Every woman is beautiful". Well, if you really think so, great! But I digress. I will not say someone is beautiful just because you say it is. I will say it only if i think it's true, simple as that.
Speaking of me, I don't think that obesity is something beautiful. Regardless of social class, age, gender, sexual orientation or any other factor. I also don't think that very tall people are attractive . They may even be "beautiful" in a vague sense , but I do not find them attractive. I never felt physical attraction for a very tall woman. It may seem arbitrary - maybe it is, but it's what I feel. I do not know why I feel it, but have no obligation to fell in any other way. I'm not disparaging anyone by not seeing beauty in them, since my personal taste is not up to anyone but me.
In my case I have no restriction on the color of skin. But skin color is a human characteristic. Just as physical type, height, eye color, hair type, hair size, etc. It is perfectly fine that someone thinks black skin is something that isn't appealing. If you want to call that "racism", fine, then I could call my preference "fatism". But the mistake here is trying to give the "heavy/bad" connotation of the term to make the person feel that his act would be reprehensible for some reason when in fact it is not.
No, nobody is forced to find someone with dark skin beautiful. Neither white skin. Or tall. Or small. Nor someone with a big nose. Or with curly hair.Nor anything else possible in the face of the planet Earth. Attraction is something that one cannot judge. Soon, people will feel entitled to "require" that the others should feel attracted by them. What kind of world would that be?