The Princess has had some male companions over the past years and certainly was not afraid of being seen, holding hands, at public places.
Recently another UK based friend of mine, who often sees members of the RF at official events, also noticed Princess Mabel with her friend in London and there was no doubt about the way they behaved that they were not 'just' acquintances.
It is wonderful for the Princess being able to lead her own independent life, away from the public eye and indeed, she is far too young to remain single.