"Are you gonna tell her, or am I?" asks Eric. Because for him, that's what's on the table: Asher is dating two people, and Eric was there first, and has a claim on Asher's sexuality, which means the relationship with Jenny is invalid.
(The actual situation is a bit more complex, yes, but I'll tell you this. In real life, Eric's version is a lot closer to the truth: that he, or all of them, would view this more as cheating than the whole beard/fake-out situation, and the reason for this is that teenagers having sexual identity issues are not like adults having sexual identity issues. It's not rare for guys to know they are gay and hide it, like Asher here, but ... Never underestimate the power of denial. I mean, it's a great parallel to Eleanor/Harold, I get that, and this story is handled pretty much flawlessly, but it's been toned down from reality, to make narrative sense and to be on TV, because the actual story makes way less sense, and involves Asher being gay some days, and not being gay other days, and making out with Eric on days he's gay and sometimes days he's not gay, and having a girlfriend the entire time, and hating himself sometimes, and being almost okay other times, and having conversations with the lacrosse buddies about fags, and then getting drunk with said lacrosse buddies and realizing they have no idea what they're doing, and most of all, Asher's days are spent doing what he has to do to survive, all the time.
And the reason for that is, your brain and your body exist on different continents, and the hormone onslaught is only the first volley of an intense 10-year war your body commits on your mind, which is why sex has driven every single one of these children completely batty, and if you're gay that's one of the weapons too: Your body starts telling you a certain thing that makes very little sense -- because sex is dirty, because everything defaults to shame, because marriage and being scared of sex and your own body are very old traditions -- which is that basically all the time, it wants to do this thing that is incredibly scary and renders you powerless, and nobody will tell you how to do right, and which involves showing your body -- which you hate -- to somebody who may or may not also hate it, while doing something complex and athletic which you have never done before. And either your body is crazy, or your brain is, and the war begins.
And if you're gay, there's a fifth column in the war you didn't even know about, an enemy inside your body, so it's two wars at once: your body starts telling you a certain thing that makes very little sense -- because heterosexuality is the default, because everything defaults to heterosexuality, because marriage and childbearing and heterosexuals are very old traditions -- which is that sometimes, or all the time, it wants to do this thing that is not the default. So either your body is crazy, or your brain is, and the war begins. And the only way anybody makes peace in this war, which everybody eventually does, is either by being raised in that rare miraculous household where you end up with the tools to actually negotiate these minefields -- lucky for you -- or by doing what Eric's done and what Serena still has to do: look that demon in the eye, and say you love it. Lucky for everybody. And the end of this particular journey is college, where you have sex constantly with everyone around you, until you are Exhausted. And then you are Old, and that's why grownups drink so much. Also why they play golf.)