I am not advocating harassment; I consider harassment if you offer someone a drink, she is refusing and you insist. Not the case. There are some unwritten rules in bars: if a woman is sitting at the bar alone, she may want a drink from some guys, not from anyone. If a woman is with some guy, she probably will not be interested and it is not a good idea to try. Showing some availability is inviting - not crying out loud "I want to be offered a drink by a tall dark stranger", but the meaning is the same. Not harassment. Females don't contact directly guys, they want to be asked and to be allowed to choose. Basic human psychology, no big deal.