HGFB wrote:God only knows what they have planned for the two Doctors.
In the spirit of RTD's minor obsession with people's "non-standard" sexualities (see Torchwood), when the Doctor meets himself, he decides he quite fancies the look of his other selves and, throwing "the laws of time" away with gay abandon (pardon the pun), has it off with himself.
Philosophers are still debating whether this actually qualifies as gay or is just an elaborate form of masturbation, and whether it could even qualify as being incestuous or not.
Luckily this didn't happen sooner when the Doctor was made human by the Chameleon Arch or it'd be even more confusing, as to whether it also technically qualified as bestiality as well.
This is partly why the Time Lords instituted the "must remain the same gender when regenerating" rule, to avoid the even more brain-rotting concept of the Doctor getting himself pregnant and being his own father and mother simultaneously.
Not that any of this has a patch on the fact that the Master is, in fact, the Doctor's son by his affair with the Master herself, when she eventually regenerates in the body of her own previous wife, Mrs.Saxon. Which pretty much ticks all the boxes of every time paradox imaginable in one go.
You see, that's what you get when you go around messing with the "the laws of time". Deeply confusing impossibly self-fulfilling plot lines that no-one understands.
Never mind. It's for Children in Need. That's not "canon", right?
p.s. No, I'm just making this all up. It's not really going to happen.

"Words ought to be a little wild - for they are assaults of thought on the unthinking."
- John Maynard Keynes