Can we use 'undesirable' as an opposite of 'desirable' in the romantic meaning of the term?
'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:
- Totally undesirable
- Completely undesirable
- Utterly undesirable, and so on.