Dr + Nyssa = entirely possible but somehow "inappropriate."
I know he normally goes for brainy/aristocratic lasses (Madame de Pompadour) rather than brassy/mouthy ones (Donna) - which was part of the fun with Lady Christina De Souza (deliberately presented as his "Irene Adler", an ideal woman / companion whom the requirements of long-term character development required him to reject).
But my hunch in this specific case was that Tegan, being Aussie and all that, "made his mind up for him", shall we say...