Wikipedia states, without a source, that this was not because of perceived effeminacy. If not for a feminine appearance of demeanor, why was Malcolm IV given such a moniker?
I have seen him referenced as "Malcolm Virgo". Was he then named "maiden" for his virginity, as he never married? Was "maiden" a gender-neutral term in 12th century Scots?
King Malcolm IV is said to have taken a vow of celibacy. So that could be the reason. Also as you said: he was never married and one of the different meanings of the term ‘maiden’ was ‘unmarried person’.
In fact in medieval times ‘maiden’ (or ‘meiden’ back then) was gender neutral. Most of the times more common referring to woman, but a man could also have been called a ‘maiden’. Back then ‘meidenhood’ was more like a ‘lifestyle-choice’ for a man and I don’t think that this name was given to him as an insult.
Sources: ['Versions of Virginity in Late Medieval England' by Sarah Salih] (http://books.google.at/books?id=xdhK06QbLHgC&pg=PA16&lpg=PA16&dq=maiden+gender+neutral+term+medieval&source=bl&ots=uyJpuypPgB&sig=cXcjW2Mq-7n9L5yk1gD58BoARsE&hl=de&sa=X&ei=AmPnU_i7JfTy7AblmYGIDg&ved=0CFIQ6AEwBg#v=onepage&q&f=false)
[www.englishmonarchs.co.uk] (http://www.englishmonarchs.co.uk/dunkeld_8.htm)