Out of universe it is a bit more complicated. Sure, giving consent is important, but there are things that are in real life beyond consent and you would normalize them by creating a universe where consent is possible if that makes sense. You can always say "he killed her but she will be reborn and they had a safe word" or "she has the body of a child but the mind of an old woman so she could give consent". Iirc this normalizing effect is the reason, even in drawn or written form, depicting sex with minors is illegal.

So is your fanfic about the romance of a beaver and a fox normalizing sex between humans and animals? Is it an allegory for mixed race or enemy families? Is it a furry fantasy? I would say in most cases, it's one of the latter and therefore not a problem. If the genitalias are too explicitly non human, maybe it's the first.

About anthros: If they are between humans and animals, I assume they have human genitalia and therefore not part of the rule. You could argue they aren't fully human and therefore do not have human genitalia even tho they look the same, but than this applies to elves, vulcans, (olympic) gods, ....

If the rule is interpreted arbitarrily in this unnecessarily strict sense, I would give you "not actively discriminated against but forgotten" which is bad enough and reading the rule that way is active discrimination, but not on the side of the people who wrote the rule but by those you enact it. I hope I'm making sense. I donated halve a liter of blood today so I blame anything on that.