It's probably moisture vapors, not "fumes". If you feel the percelain part of the toilet and also the water in the toilet, it is very cold. That water is coming from out of the ground. As seasons change and the ground gets colder, so does the toilet, the water, and the air that is in the toilet bowl.
Your urine is heated to body temperature. When it is released from your body into the cold air of the toilet bowl, you will see moisture vapors. It is the same concept of an outdoor hot tup/jaccuzzi that is heated when it's cold outside. It emits vapors. Or, like when you pour a hot cup of coffee and it comes in contact with the colder air... same thing.... vapors.