Depends on where the hospital is and who runs it.
Many hospitals only allow immediate family members into an ICU. Long time partners, regardless of sexual orientation....do not count as "immediate" family. Nor is that person's opinion regarding the care of the patient, relevant. It is also often, that only the partner really knows the wishes of the patient regarding their medical care.
When M was in the hospital 4 years ago...I had his medical power of attorney that was signed and notarized 3 days before admission. Without that, I would have been blocked from any decision regarding his care....not only by the hospital...but his children could also have blocked me. As it was...with the exception of him signing himself into the hospital, all decisions were made between his doctors and myself. Including who his visitors were....and weren't.
His doctors were aware of our relationship status, but the nursing staff wasn't until a copy of the POA was included into his chart. I was also initially asked to show my driver's license for verification of who I was, and they made a copy of it and included it in his chart too.
His admission was kind of an emergency, but if I hadn't had that POA....and it had been an emergent situation like a car accident or heart attack....I would have been kept out of the ICU, decision making, everything. And I'm the one who knows him best....not his kids, who would have been the ones making the decisions.