If I've never said it this way before, I'll say it now: if you don't believe in animal rights, the argument from marginal cases is your biggest challenge.
Take Robert Garner, who co-wrote The Animal Rights Debate with Gary Francione. Garner is an advocate of animal welfare reform, and he certainly thinks there is a great deal wrong with the current state of animal exploitation (honestly, this is nearly nonsensical to deny). However, he does not take a strong rights position, merely claiming that animals have an interest in not suffering (and not an interest in not being used or killed.)
At one point he gestures at the argument from marginal cases, indicates that it's very controversial in the literature, and suggests that he has reasons to doubt its validity. Of course, he fails to actually spell out these reasons.
Essentially he claims that because of the diminished capacities that animals have compared to humans, they lack the ability to have an interest (and therefore a right) in continued existence. This is a bizarre claim on its face, because its hardly implausible that to go on living is prima facie a good thing for anyone with a chance at a decent life. Nevertheless, this argument implies that there are some humans who have diminished capacities and thus also lack this right. Most people will regard the idea that a sentient human doesn't have a right (in some sense) to their own life as vile.
What of the option of denying the argument from marginal cases? Well this would result in arguing that species is a moral criterion. And it's very difficult to see how this argument could plausibly be made, without providing equal justification for racism, sexism, and indeed, ableism.