well said, my ex was "pretty" (big busted), and she had lots of
suitors who did that, like popping out of nowhere saying stuff like
"oh i liked you since J1, can you give me a chance?"
i took it badly initially, which was bad because it shows how badly
i took it. i msged the guy back, ended up with a "macho" SMS war
which had no meaning or did not help the situation.
look at it this way, if you're too bothered by the msgs, your gf
would most likely get upset, because it shows how you think such
msgs would sway her feelings for you. so don't do that. Fireice is
right, have more confidence, and trust in her that she won't do
anything about them. there's nothing much she can do when some guy
likes her right? and it's worse when it's her friend, because you
can't expect her to completely break ties with her friend just to
pacify your jealous thoughts? put in another way, if the reverse
happened to you, i'm sure you'll appreciate it if your gf trusts
you and not think too much into it. be confident, your gf is not
with you for no reason, and won't be swayed by such "advances". and
the fact that she told you tells you that it is indeed so.
of course it's another case if your gf encourages such activity.
but put it in another way, some girls are (not saying yours is, but
just consider it) flattered by such msgs, but they won't do
anything serious about them.
bottomline, it shows how desirable your gf is to others, and that
you're the envy of other guys. so take it in your stride and show
her the gentleman you are! 
