This was brought on by a question relating to how the Doctor would respond to gay marriage, to which Moffat very accurately pointed out:
First things first, the Doctor simply does not care one iota about which way a person swings. The people that he travels with or the friends that he has made in his 900 years (and then some) of travels are based on their morals and their ethical viewpoints, not on who they sleep with at night.
Whoever asked the question clearly hasn’t been watching Doctor Who at all. The Doctor has come up against all sorts of characters in his meanderings and not one of them has had to explain their sexual preference to him, simply because he doesn’t care.
When he met Captain Jack Harkness, the Ninth Doctor knew from an early point that the good Captain didn’t only like attractive blonde shop assistants. Did it bother him? No. He explained the situation to Rose and moved on. The Eighth Doctor had some idea of Izzy’s sexual orientation but let their friendship blossom rather than ask questions that weren’t his business, didn’t concern him (see various Eighth Doctor comic strips from 1996 to 2005 in Doctor Who Magazine) or about which he knew really wasn’t of any earth-shattering importance.
The point is that sexuality isn’t an interesting point in Doctor Who; it’s not something that we care about. Actions speak louder than words and if you’re saving the Universe it shouldn’t matter whose bottom you stare at on a Friday night.
The answer to “who is bisexual in Doctor Who?” is, of course…
When Moffat continued the conversation it was with another fan…
…but to be fair, who really cares?