Haven't read all of thread,
But it is a bit odd, don't get me wrong I think having babies is exciting and lovely and all that and would wish anyone well, but it's quite school playground to say they know the sex but not tell anyone.
Remember the kids at school "I've got a secret and I'm not telling you". For me it's all a bit like they want to hold your attention for as long as possible or for you to keep on asking them or something.
I had a friend who did this with their baby's name, quite ridiculous.
Had it poseted all over facebook for weeks that her and DH were deciding on names but not even telling what the choices were, just seemed the really love stringing everyone along and getting all the attention.
I'd love to just reply to someone who says we know what sex it is but not telling you by saying "well I don't give a shite anyway, I'll know when it's born".
Just joking though really! Just be good to see their face.
It is annoying, but not the end of the world, if we don't care what sex it is it doesn't really matter that they don't tell.