Snipersmum - I am really grateful to you for starting this thread as I am going through exactly the same thing.
We have one of each (we used the "Choose the Sex of Your Baby book" the 2nd time round) and had only ever planned two. We didn't try and determine the sex this time round but I spent the first 20 weeks of pregnancy visualising my three children, DS, DD and DD2.
I tried to find the gender out at 13 weeks but we couldn't see. At 21 weeks, I was told it was a boy!!
I've spent the last 6 weeks grieving for my "lost" DD2. I can't bear the thought of throwing away DD1's baby clothes as I still hanker for a DD2.
I'm not sad about having another DS - DS1 is a star and he is so happy to be having a baby brother and I love little boys. But I'm still grieving for DD2 and feel of women currently PG with a girl, or who have two or more daughters.
Trouble is, I'm 43 and DH would proably leave me if I suggested having a fourth (he's quite a lot older than me, and I'm pretty mature!). Plus, a fourth might well be a DS3 anyway!
This is the first time I've been able to express this and I have cried whilst reading this thread.
So Snipersmum, I feel exactly the same as you. I am starting to come to terms with it now and am hoping that No 3 will leave me so busy that all thoughts of hankering after another will rapidly dissappear!! I know I will love him, but wonder what a DD2 would be like......