I have loved reading this thread as i am in the same boat. I have ds1 (nearly 9) and ds2 (2.5yrs). I often think about having another one. I would love to have a girl but wouldnt want to go through the dissapointment of another boy. Dont get me wrong I love the two I have got and have never once regretted having them but I have always dreamt of having a little girl, I have always had such a close relationship with my mum i would love to share that experience with my own little girl, also little boys dont really appreciate the same things us girls do
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Every now and again dh will bring up the subject of having another, i think deep down he would like one but just like me he is not 100% sure about it.
On the practical side we live in a 2bed damp riddled council flat (we are trying to move but thats a different thread)so there isnt the space and i hate the thought of my boys living in this let alone another baby, if/when we get a mortage money will be tight, Ds2 is now at a nice age where he is easier to look after ie amuses himself (sometimes)and doesnt need feeding every few hours. When ds2 is 18 we will still only be 44 so plenty if time for dh and I to enjoy oursleves before we get too old. Also after having ds2 i have found it hard to get back in shape and now that after having baby no.3 it will be even harder. Being overweight is not something i enjoy and it gets me down a lot.
On the emotional side I love being pregnant, no morning sickness fairly easy labours although no.3 would probably be totally different to that. Nothing beats the feeling of holding your newborn in your arms and babies are so gorgeous. I already had ds1 when i met dh, hence the big age gap, and I know dh has always wanted 2. He classes ds1 as his own so technically he has got his 2 but I would love another baby with dh, its hard to explain but it would make our relationship feel complete. The excitement you feel when you get that big fat positive when you have been trying to get pg for a while. And dont forget the fun you have while trying 
I know that if we do decide on no.3 i dont want to have such a big age gap between ds2 and the new baby but then again we have got so much planned for the next 2years that having a baby between now and then wouldnt be practical. In a way I wish the decision would be taken away from me and the pill would fail, that way we would have to live with it but then again at the beginning of the year i missed a period and had never felt so scared, it was like being 19 again and telling my parents i was pg with ds1. With ds2 we had had a false positive a few months before and after getting our hopes up for nothing we decided to actively try for ds2. I suppose the fact that the scare at the beginning of the year hasnt prompted us to try again should make me realise that a having a baby now just isnt right for us and we obviously both know it. I just dont want to have to face the fact that i may never have another baby. I know that at 28 I still have a few years left to decide but i have always known i dont want a baby after the age of 32, no particular reason just personal preference.
Who would think that deciding how may children to have would be such a difficult decision? Choosing to have 2 children was such an easy one to make compared to this.