This is why I'm not a fan of fitting unconventional games into conventional genres.
Portals can be used offensively, and this is in fact required for certain achievements and puzzle solving methods. The level where you first encounter sentry guns is a very clear example of this: while it is technically possible to pass the level without using portals to "destroy" any of the sentry guns, it is extremely difficult.
Despite this, I wouldn't call Portal a "shooter" either... nor would I call a game like Mirror's Edge one. I'm simply saying that, in black and white terms, they both fall within the necessary criteria for being classified as FPSs.
A "correct" term for Portal would be something akin to "first person puzzle solving game," but such a genre classification does not exist in conventional terms.
Really, though, we're talking about trivial semantics here. You say tomato, I say tomato.