Opposite Gender Protagonists/Analysis

Audience Immersion
It may be a bit hard to believe, but girls do, in fact, exist on Earth, and they seek entertainment. Any media which appeals only to boys or girls won't cover everyone in the world, and will lose out on half of their potential demographic. On television, Ratings are everything. If you don't want your shows to get cancelled, you must keep your ratings up. If you want to keep your ratings up, you need to appeal to as wide of a demographic as possible.

How do you appeal to that demographic? One of the easiest ways is to make your protagonist a member of that demographic. Appealing to people who with No Social Skills? Make your protagonist someone with No Social Skills! Appealing to people who likes science and to think critically? Make your protagonist a Geek into technology! Appealing to teenagers? Make your protagonist a Kid Hero!

Similarly, if you want to appeal to girls, make your protagonist a girl. If you want to appeal to boys, make your character a boy.

If you want to appeal to both, use Opposite Gender Protagonists. By ensuring that both genders have some character to relate with, both genders are more likely to stay and be invested. Thus, both protagonists should be equal and work together, instead of against each other. Doing so means that, to both genders, the work is more satisfying to watch and they'll stay invested for longer. It is unsatisfying to see one character constantly dominated by the other, or become The Scrappy and contribute nothing to the team. Having the two characters take center stage means they are a larger target for audience members to associate themselves with.

As a bonus, sometimes the male demographic can be attracted to the female character, or vice versa, further increasing audience engagement.

In Universe Functions
Any trope in a story can accomplish three things, in order from most to least common: advance the story, develop the characters, or build the world. This trope is good at achieving the former two, and especially good at the second point.

Advancing the Story
Having two characters means more opportunities for the characters to advance the story themselves. Perhaps you're looking for a MacGuffin Girl or Phlebotinum Girl, along with a Hot Blooded boy. The two characters serve two very different purposes, with very different characterisations. The former takes the story to the next stage, but need support. A MacGuffin girl needs to be found first. A Phlebotinum Girl is often a Rei Ayanami Expy and thus need a lot of emotional support. The latter gives the former the help they need. Not only does this add to their characterisation, but it also gives the girl the push needed to advance the story, and to serve her critical purpose. Maybe the girl has the MacGuffin and the boy is about to go full I Will Protect Her. Once again, we build character, but the girl's MacGuffin is crucial to advancing the plot. However, this also means the girl is prone to getting captured and must be rescued by the boy. Or maybe it's the other way around. We have an Action Girl who needs to rescue a shy boy. All of this advances the story due to the natural interactions between the leads, but also note that it builds character at the same time, which leads us into...

Developing the Characters
The trope places two characters at the very center of the story, so the audience expects that the two will have some sort of interesting interaction. If they lack chemistry, or just sit there in silence staring at each other, then why are they portrayed at the center stage in the first place? Being the leads, and having to go through the difficulties of adventure, the bond between the two characters naturally strengthen. The usage of only two characters, with them being equally important means the formed bond is stronger because there are less characters to dilute or complicate the relationship. The bond is further strengthen as, being the main characters, the two get a lot of screen time, and more opportunities for developing their interactions. Having the two leads being opposite genders paves the way for an easy romance in the future, which, once again, increases audience engagement.

Oh, speaking of romance...

Why the genders?
At this point, you may be wondering where the "Opposite Gender" part of the title comes from. Currently everything under the "In Universe Functions" section can apply to any sort of duo taking center stage. The reality is, the characters don't necessarily have to be of opposite genders. The trope can be tweaked just a bit to provide for a same-gender duo, but why do so many people choose to do this instead of a same-sex duo?

The thing is, it works. Have a boy and a girl, and audiences can relate to the main characters. Chances are, they have experienced some sort of man-woman relationship, but haven't experienced some sort of homosexual relationship. Tropes Are Tools, and this is just another tool in the box. It's a tool designed to reach as wide of an audience appeal as possible. Yes, it is true that the LGBTQ+ movement is growing, however the fact is that its members still remains a small fraction of the general population.

Of course, the characters don't even have engage in a romance. In that case, the trope is used simply because that's how it's been done in the past. Tropes provide audiences with a sense of familiarity, and this trope is one of the most powerful way to do it, combining the familiarity of a girl, boy, and protagonist. It's nothing ground-breaking, and that's the appeal. It's familiar, like coming home after a long vacation. Sure, there's nothing new, and you might get bored quickly, but you must admit that constantly coming back to a place that never changes gives a feeling of certainty and thus comfort. Having same gender duos deviates from the formula, giving audiences a fresh, but ultimately foreign feeling. This is the reason why The Princess Bride is so popular. It doesn't try anything new, but it develops existing stories and formulas well, giving audiences that sense of comfort.

In conclusion, the characters don't have to be opposite genders, but they are because that's how it's done in the past and it's relatable to the majority of the population.