I have seen it and boy do I disagree!! And you’re exactly right! It’s the exact same rhetoric that cis people have been using at why cisgender is offensive! (For the most part. I’ll get to that in a second.)
All allosexual means is just “not asexual” therefore anyone of any sexuality that isn’t on the ace spectrum is allosexual. That’s it! It’s not offensive. It’s not a slur. And it cannot be oppressive because asexuals are the oppressed minority. (Wow look at this academic article about it.)
As for the other arguments I’ve seen…. People say it’s not a good term because it groups straight people and LGB+ people (who aren’t ace spec) together. And they say that’s a problem because they’re not the same. And yes. They aren’t. But when talking about allo spec vs. ace spec issues…. they are. All allosexuals can be oppressive to asexual spectrum people. That does not mean that those allosexual people cannot also be oppressed for their sexuality. But it does not mean that they are immune to oppressing people of other sexualities as well. (ex. gay and lesbian people oppress multisexual people all the time.) Intersectionality is a thing. Oppression is not an all or nothing thing. Just because you are oppressed in some ways does not mean you are immune to oppressing people in the same or other ways.
I am a white, gay, ace spec, nuerodivergant, able bodied, middle class, college educated, agnostic, trans man. See those three bolded elements? Those give me a lot of privilege. (other things too, but not just from looking at me.) I can be really oppressive to others if I don’t watch myself. While there’s still a ton of things I can be oppressed for. On the surface… none of those things are seen. If I’m just walking on the street I am a well dressed white male, I’m probably really safe. Now if I was in a setting where any number of those things I could be oppressed for came to light, that’s a different story. But I still have privilege.
It’s the same way for allosexual people. Just because they are oppressed in one way does not mean you are privileged in another. Intersectionality is a thing.