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.
https://en.oxforddictionaries.com/definition/undesirable
undesirable
ADJECTIVENot 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?
Best Answer
'Unattractive' would be more often used than 'undesirable' if you are talking about a person. 'Undesirable' is more often used to describe an outcome or situation or result.
If you were to go so far as to say 'undesirable' about a person in a romantic sense, it would be better to add to it: