Well, Troi appears to be a single gal.
And it's not as if Alexander would specifically need a female protector: he's done breast-feeding, supposedly, for the time being. A male role model might do him more good at this point.
From what I remember of the episode, I think the implication was that Worf understood that Alexander needed someone who would genuinely care for him and provide a nurturing environment. Several episodes had been implying that Worf saw Troi as a good possible mate for him, and by extension a good mother for Alexander?
That episode had much worse issues IMO, not one of my favorites.