I really think that it boils down to the community.
In some areas it is OK to great or talk to the opposite gender, in other communities it is unacceptable to do so.
Which way is the right one. I don’t think that it is up to some online posters to decide.
That said, there is no issue with people of the same gender to great each other. I feel sad when I see someone ignoring my greeting, I look at them as an not (yet) fully trained individual.