Well, I've only seen a few here and there of the old series, and getting better bit by bit, but I think I would attribute what I've seen mostly to the actor flirting with the girls rather than the Doctor flirting with them. And considering the change in times, where nowadays we're not as afraid to show a more committed character onscreen, the potential for intimacy is much higher. But then, that's a character thing. A character like Rose would be more content with the intimacy factor, being a much more emotional character than, let's say River Song. River is a much more physical and carefree character, and wouldn't let something like a little biology or rules stand in the way of her fun.
From what I've seen of Romana and Sarah Jane, however (and it's not much, I'll admit), both of them either seek to be the Doctor's equal rather than his love, in any form. And while yeah, they might seek a physical relationship once the emotional is present (which they probably would), the logical side of them might get in the way. Trying to be the Doctor's equal means having to fight with that ego, and that's no small thing.
And while he might avoid it, the Doctor certainly knows what to do--even if it's been awhile.