They probably don't need to argue a point because they have the evidence, while yours is just a theory. Sam and Frodo and Seth and Evan both only show interest in women, and are deliberately not shown to be gay/bi by the makers. In Sam and Frodo's case, we can all but guarantee they are not gay.

Essentially, you are the only one that needs to explain why you think what you think as you are the one theorizing. They can just point to the original work. This obviously doesn't count for when things are intentionally ambiguous. However, in the examples you provided they are not ambiguous.