Portals defy known physics on so many levels that it's hard to get in to.
Fixed your post.

Any hypothetical portal that can exist in the future absolutely does not defy physics, its mere existence is absolute proof.

The problem with sci-fi stuff like this and debating whether such things are possible is that there's no clear definition, so to simply say "portals defy physics" is a pretty broad statement. I can prove it wrong by pointing out that a large imposing doorway is, by definition, a portal, and large imposing doorways don't defy physics, in fact I'd go as far to say we have an almost complete understanding of the physics of imposing doorways.