What is the difference between the following statements?
Most men have peculiar manners.
Most men have manners peculiar to themselves.
Peculiar by itself simply means odd or strange, with a touch of eccentric. Your first statement is saying that most men act strangely.
Peculiar to oneself means unique or distinctive, not necessarily odd. Your second statement is saying that most men have habits which are easily distinguishable from other men's habits.
...seems like reverse sexism to me. (A statement like only a rank femininist would make.) But in this use the word 'peculiar' definitely has the connotation of "strange", "odd" or "weird."
"Peculiar" here sound like it means "unique" or "able to be distinguished." It's not saying the manners are weird, but that they are distinctive and individual. You could tell each man apart -- even if they looked nearly identical -- because each one has their own set of mannerisms to distinguish each one from the other.