Hm, well Superboy had kind of a reputation for being kinda of womanising I guess (hitting on female reporters etc. when he first appeared, Knockout, Wondergirl etc. etc.), so maybe his concern was a combination of the realistic 'I don't want my daughter to be another metaphorical notch on his bedpost' with the superheroic 'He's unreliable and his superpowers mean that me might hurt her by mistake'.