Let me preface this by saying that some of these comments should be taken with a grain of salt. That is, someone who said she never had any problems at all was probably extremely lucky or didn't consider cat-calling to be a problem, and someone who says never travel alone either had really bad luck or had a low threshold for harassment. Most women definitely found that being in Southern Europe was much more difficult than Northern Europe. In Mediterranean countries there tends to be lots of cat-calling that sometimes results in being followed. Some men will even go so far as to touch you.
More respect seems to be shown in the North.
I'm glad I traveled alone. Pretend you don't hear. There were many other women I met along the way who were also traveling alone. It was great to team up for a day or two. Men all over the world can be dirty or dangerous.