I registered just for this - hooray. \o/
I'd like to address the "sexuality doesn't come up in games", "sexuality isn't relevant in games" and "you can interpret the characters however you like" arguments all in one go.
Heteronormativity in most media is very very subtle at times but leaves no room for interpretation (except in the fanfiction world
). Examples I've often seen in these arguments are characters like Marcus Fenix, Link, Master Chief, Mario, Gordon Freeman, and other protagonists who don't directly reference their sexuality. Somehow we're meant to project ourselves onto them to make them whatever we want them to be. These characters are often described as asexual or we're told their sexuality can be interpreted however we want, but clues in the games themselves steer toward them being straight.
Marcus and Dom in Gears of War - yes yes, we've all heard the jokes and the developers had a hand in that in the first game when they put in co-op achievements like "I Can't Quit You" but both characters have only shown heterosexual behavior in their games (at least up to the halfway point of Gears 3 - I still haven't finished that game). Dom is searching for his wife, and Marcus is heavily implied to have a thing for Anya. This doesn't mean they can't both be bi, but there's no evidence for that - their own relationship with each other is portrayed more as a brotherly love.
Link and Mario - This one is easy. When have either of these character rescued a prince? Hetero norm.
Master Chief - This is probably the loosest example because Spartans for the most part appear to be asexual (until Halo 4, but those are Spartan 4s) but there is a bit of heteronormativity here in Master Chief's attachment to Cortana. If Master Chief's A.I. had been someone like Roland (Halo 4) from the outset, would we see a bond like we do between Chief and Cortana? Likely no. Yeah, you could probably make the argument that since Cortana is a digital clone of Doctor Halsey (the only mother Master Chief has ever known) the relationship isn't necessarily sexual in nature, but their dialog implies otherwise. Actually a better argument would be for objectophilia but that's all based on whether or not you consider Cortana a person.
Gordon Freeman - silent protagonist, we never see his face in-game, but through the actions of the other characters were supposed to feel an emotional attachment to Alyx Vance.
Just because the above characters never shoved their tongue in a woman's mouth doesn't mean they're not written as heterosexual or from a heterosexual perspective. And as for why it matters, it's about some gamers' ability to relate to these characters and the discomfort some feel playing as heterosexual characters in every single game. Also when a group is underrepresented in a genre it can feel like they're not welcomed or they don't belong - especially to a group that's not allowed to marry in most states and constantly has people telling them that what they do is a sin. So yeah, it matters.
Sorry for wordy wall o' text. If you read this far, have a cookie. ^_^