I think its all BS. I've got plenty female friends, just as many as male friends. I have a gf for 8 years . No hassles . Its about having respect for those people you call friends and not seeing them as objects you wish to bed.
My gf has lots of friends of each gender as well.
Firstly, how old are you? I'm asking because relationships between the genders change quite a bit depending on your age group.
Secondly, are you attracted to any of your female friends? I'm not saying you are OBJECTIFYING your female friends. I am asking that if your GF wasn't in the picture, would you find any of them reasonably sexy/interesting enough that you would consider them an option as a girlfriend? I'm assuming yes, unless all your female friends are ugly.
Thirdly, do you think any of your female friends have a crush on you?
My 2c in regards to the topic is both "yes" and "no".
If both man and woman have enough in common, but don't have any kind of sexual attraction (from one or both parties), yes they can be friends. In both men and women, this usually involves them keeping their sexual attraction in check if they have been told the other party doesn't feel the same way, or if the other party is attached.
If both man and woman have enough in common, AND have sexual attraction BUT are both in monogomous relationships, yes they can be friends. Apply alcohol to a man and woman who are in this situation usual ends in sexual behaviour between the two parties involved.
If the man is homosexual, yes they can be friends.
If a man and woman are both single, and both have sexual attraction towards each other, no they can't be friends.
As someone who is currently single, my current objective is to find a gf/fwb, therefore the following applies:
Any new female (i.e. isn't a friend) is a potential girlfriend. Do I have her physically attractive? Yes/No. Do I find her interesting? Yes/No.
If girl attractive but uninteresting, she is a potential fwb.
If girl attractive and interesting, then she is a potential gf.
If girl is unattractive but interesting, then she is a potential friend.
If girl is unattractive and uninteresting, then she won't be more than a possible acquaintance.
You can swap out girl with boy for girls. I know most girls want the security of a bf, but you do get the odd one who just wants to shag without the relationship side.