"First boy I ever fancied was called Rory...Actually she was called Nina. I was going through a phase. Just flirting to keep you cheerful."
^This is what we've got a problem with. This line.
Whether Moffat meant to be flippant or not does not stop the line being harmful. The gag of calling Rory 'Nina' for the rest of the episode would have held if written differently without the 'phase' bit. It doesn't do us any good towards debunking the myth that all bisexuality is just a phase if a major show portrays the issue as such, and so casually, for a gag no less.
We're not in a place for it to be used comically yet, with bisexuality being misunderstood with lots of negative stereotypes by a wide portion of both straight and gay populations. We're not there yet, whilst bisexuals still have the highest rate of depression and suicide in LGB.
My dad is a liberal inclusive person who has no problem with non-heteronormative sexualities or genderqueer identities, but even he as a middle class straight white man felt obliged to tell me it was fine if it was just a phase when I came out to him as bisexual. He should not have felt he needed to say that, and it is because the culture still marries the two things together (bisexuality and going through a phase) that my own father made me cry by saying the one thing I was shit scared about my parents saying in response to finding out about my bisexuality, which he never intended to do; he was shocked and upset at my reaction, never having intended to be negative. We are fighting everyday ignorance like this.
I am no longer scared by that response, simply because I know it to be false with no shadow of a doubt - my bisexuality is not phase. But other young people, like I was before I was completely out, are growing up like I did, surrounded by this misconception that straight and gay people hold that bisexuals are just going through a phase and will end up on a 'side' at some point.
This is generally not the case, and the assumption that it is is damaging for people who are bi and uninformed except for these culturally accepted perceptions. It's not a surprise that lots of bi's get depressed and suicidal if their culture, supported by popular media, tells them that what they are feeling is not possible, or true, and is definitely going to change.
I assume Moffat was just trying to be funny, but he cannot go around using queerness in his writing without being properly informed on the impact of how he is writing it. Ignorance of the baggage behind 'just a phase' does not excuse him from the damage it does by perpetuating the misconception. It may not seem like a big deal to those who do not have to face a society telling them that what they identify as does not actually exist, but 'just a phase' is not just a phrase. It is a hurtful, harmful weapon that should not be tolerated in popular media until we're in a very different place culturally. In today's campaign for understanding and acceptance of bisexuality, it is the equivalent of 'it's unnatural' or 'it's perverted' was in late 20th century campaign for the understanding and acceptance for homosexuality.
Homosexuality was seen as a social problem that encouraged promiscuity, and now that has shifted to bisexuality. Homosexuality was seen as something that could be cured, and now bisexuality is also seen as something transient.
We are at the third stage of our battle for equality - first G, then L, now B, and once bisexuality is as easily accepted as homosexuality, the T's will come to the fore. But for the moment, it is our turn. And Moffat is supporting the opposing side, even if it is unwittingly.
Further to my thoughts, here are some already articulate discussions of it that I recommend reading:
"It is a phrase with a shit ton of baggage and someone thinking they can get away with saying it on one of the most widely viewed shows (a family show nonetheless) in the world is rather disgusting."
"reinforcing about every parent's mindset that their gay/bi/pan child is just “going through a phase” isn't good."
"do you think it was added in so that bisexual young people could be reassured that what they’re feeling is legitimate and normal? Nope! It’s added in so that she has that saucy bit of forbidden naughtiness in her past that in no way affects her sexuality now."
"that kind of writing treats bi/pan-sexuality like a joke, like a funny little quip, like "oh look we're so progressive this character had a fling with someone of the same gender, they're so quirky and cool," and it trivialises something that is actually kind of a big deal for a lot of people."
"she says it to rory, to flirt with him. cause girl on girl is only a tool to turn guys on right?"