My mum now calls me...

[Ms Grace]

my friends & I have a theory that when a guy calls a female mate it ends all hope of him sleeping with her ever.

Haha, probably true!

Is it okay to be called "A Sleila"..?
(Remembering Crocodile Dundee)..

"Sheila" is pretty outdated these days, and was more of a third person reference ("check out that sheila") or as part of an insult ("you crazy sheila!")... unlike "mate" it doesn't get used as a term denoting interrelation.

There was a woman at my uni many years ago named Sheila Shaver...and yeah, the juvenile in me couldn't help but be boggled by that combination of words.