Imagine some of that sweet Portuguese urine transported with horse and cart in the warm summer days over many miles right to your mouth
But also imagine how bad the normal smell must have been if old urine smell actually was better
I want to pitch in that the saliva production is the key. People with good saliva production don't suffer as much as those with bad when not brushing. The saliva production tends to drop off a little with age but I'd guess young people and teenagers could still kiss without much issue. Some smell wouldn't be a turn-off, imagine people barely showered pre-toothbrush either...