The suggestment was that Susan Chesterton was conceived in the TARDIS during the Hartnell era by Ian and Barbara who doubtlessly loved each other more than enough to get married if only they wen't always running through long metal corridors 14 hours out of any given day.
Ben and Polly might have been doing it too, but she seemed like a good girl who would want to wait.
I doubt Jamie knew what sex was, and Stephen Taylor seemed far too... There's a line in the pilot to X-Files: "Have you ever been for a joyride in a flying saucer? Afterwards sex seems trite". Regardless you don't become an astronaut until after you've lost interest in sex... Although Sarah Kingdom might have forced herself on him, and Katrina might have supplicated herself to any of the Gods living in the blue temple.
Dodo would be on the pill (Was it legal in 1965?).
Which only leaves Harry and Sarah Jane or, call me crazy, but the way Tegan acted around Turlough, it's almost like she wanted to dunk his pigtails in an inkwell, there was far too much emotion going on between them for it not to be confusing and misdirected.
So many, many potential timeheads.
