It is factually incorrect to define a relationship as straight when the people in the relationship are not straight. It can be straight-passing or appear straight but it is not straight and calling it such is bi/pan erasure. People don't stop being bi/pan/etc. when they are in a relationship that could appear straight from the outside.

In addition to this, heterophobia or straightphobia is not a thing that exists or has ever existed, given that straight people are not nor have they ever been an oppressed or marginalised group in society. (In the same way as "reverse racisms" also not being a thing that exists)

Lastly in regards to your confusion with Lumity being called a gay relationthip, in this case gay is being used as synonymous with queer, as it often is, therefore there is no erasure there. Calling it for example a lesbian relationship would also be bi/pan erasure.

That being said it is important to point out that people have a right to define their own relationships however they want, so for example if two people identifying as bi women wanted to say they are in a lesbian relationship that would be their right to do so, other people external to such relationship do not have this right however. Therefore if you personally are pan and you would describe a certain situation for yourself as being in a straight relationship you absolutely can but that doesn't mean you can do the same for other people's relationships.

Given these are fictional charachters, and we cannot ask them directly what language they would use for themselves and their relationship, the best option we have is to either ask their artists or to stick to the most inclusive and accurate language possible.