I think what matters here is context. Sexual dimorphism is frequently used as a stick to beat trans people with, so that's clearly a problem and needs to be called out wherever it happens.
In the context of animals other than humans, though, they don't have the concept of gender that we do, so I don't think you can really call that sort of language transphobic in that context. The language used may run close to what can be considered transphobic, and I think that's what SnakeFighter was getting at, but unless someone came in and started applying it to humans as well, then it's not as harmful. If they did, then you can call them out on it.
I don't think it's a problem to have the discussion. Just need to be a bit more conscious of the context of what's being discussed, I think.
(also sorry if I'm late with this and the discussion already moved on, but I typed this up on mobile so it took a while.)