Considering the use of 'desirable' in a romantic sense to mean 'attractive' as in
"She looks so desirable in that golden dress!"
is 'undesirable' appropriate to be used as antonym?
She found him undesirable.
Not wanted or desirable because harmful, objectionable, or unpleasant. ‘the drug's undesirable side effects’
Although the above dictionary does list 'unpleasant' as a meaning, I wonder whether 'undesirable' is (not) a good choice of word to mean 'unattractive' or 'not desirable' in the romantic sense?
Edit: Based on the first answer,'undesirable' seems to be an example of a type of word that is constructed by adding a negative prefix to another word, but no longer serves as its most common antonym (Un + desirable = undesirable, but the most common antonym of desirable is now apparently 'unattractive') -- can members quote a few other examples?