He says he wants a baby sister, but....

I wasn't supposed to be able to have kids. I wasn't even supposed to get pregnant, much less carry a baby to full-term. But for some unknown reason, my body not only decided to produce enough hormones to release an egg, but also to produce enough hormones to make my body viable enough to produce a miracle. Women with my condition, rarely get pregnant without medical intervention, and if they do, the rarely make it past the first 2 months of pregnancy, so whether it was a fluke of nature, or truly divine intervention, my 6 year old son is a living, breathing miracle.
Yet, he insists that he wants a baby sister. Even after telling him there probably won't be any more babies in our house until he grows up and has kids of his own, he is adamant that he will be a big brother soon. How do you explain this to a child?
But this really isn't what this is about. It's about the 6th sense that kids have whenever their parents are suddenly doing something that doesn't involve them. We could lay in bed all morning and not be interrupted once by our son, but as soon as we even think about becoming intimate, he is bursting through the door and jumping on the bed and asking what we are doing. Unfortunately, at this current time in our lives, we live in a one bedroom apartment with a section partitioned off for him, so intimacy is not something we see very often in our lives right now. But as my husband noted, "He says he really, really wants a baby sister, but.... he's not letting it happen."