Babies have no sense of personal hygiene, but everyone seems to love them. : )
In an adult, I would find if almost, but not completely, impossible. It would depend upon how much I loved them.
I’m guessing that there are some medical conditions that cause awful smells, but people who have spouses with these conditions often put up with it.
In the absence of being a baby, or having a medical condition, it would be hard, but not impossible, to stay with that person. Loving them, is a slightly different situation, carrying on a physical relationship is quite different. You can gently help people to smell better (if it’s just hygiene and not a medical situation). Someone who resists this help probably has other problems that most people are not equipped to deal with.
I had a boss once who never used deodorant. He was a nice enough fellow, but he stank. Not worse than anyone else who doesn’t use deodorant, but it was noticeable. No one said a word to him, because no one wanted to hurt his feelings. One day, we were all talking about whatever and he said, “I’m lucky, I don’t put off a bad smell, so I don’t need to use deodorant.” The bunch of us all looked at each other with surprised looks. We still didn’t want to hurt this guy’s feelings, so we didn’t say anything. At that point, we realized that he must have had a bad sense of smell. He smoked like a chimney, and I’m pretty sure his sense of smell was shot because of that, but at least we now knew what was going on. Luckily he just had regular B.O., not that kind of smell that seems like a dead body has taken up residence in your unwashed private parts, which is a smell like no other. That guy, I could not put up with. I could also not deal with anyone who smelled like feta cheese that has been left out.
With regards to the medical problems that cause bad smells, I liken it to smelly dogs. I have loved a lot of smelly dogs in my life : )