I try to apply a rule based on what I observe in places like The Underground.
Public place eg train, underground, bus, theatre etc. People want their space and privacy, so I would sit in the next seating bay if free, or if the carriage was filling up more, leave a seat free between me and another person, and then when things have filled up so much that this is no longer possible , sit next to someone.
The word I have from women friends is that they feel a background wariness in public, not from anything personal from other persons, but as the sex which often has less physical strength or agressive instincts, the risk factors with unwanted attention or worse are increased, and as a result of this, feel much more comfoirtable having at least one empty seat between them and someone who is not anyone they know.
It feels intimidating if in a fairly empty carriage or theatre someone sits in the next seat. It looks as if the person has chosen to sit there in preference to all the empty seats, and as a precaution they may decide that the reason the person chose that seat is to be next to them nis to be next to them, and when it is someone they don't know, regardless of whatever that person is intending, it feels uncomfortable and a bit threatening.