"son is a son till he finds a wife. A daughter is a daughter all of your life".
Can you explain what you mean by this Happy? When a man gets married what is he supposed to do. You have just effectively told the op, mother of two boys to count herself lucky because she's on a clock with the two adored children she already has. It is a way to set up an army of evil MILs though I suppose; tell them while their boys are babies that they must cling on while they can because an evil wench will steal their precious babies away. I must tell DH and his two brothers that frankly, they are letting their sex down. We see MIL all the time. Several times a week. We go there and stay at weekends, we go on holiday with them and we speak on the phone most days. DH's sister on the other hand has moved to the other side of the country. She is married to a woman though. Maybe that's the mitigating factor. 
Strawberryshortcake "I don't get how anyone can be 'devastated' by the sex of their baby. Surely people know when they get pregnant that there's a 50-50 chance of having either a boy or a girl. There are people out there who would give their right arm to have either, u should count yourself lucky." I'm not singling out you or your post as many other posters have expressed the same sentiments but you've included quite few of the common kneejerk reactions so I'm just going to respond to that in general terms iyswim.
You don't know how people can be devastated. You admit that freely. That's the crux of your problem. You haven't been there. You haven't experienced it. That doesn't make it untrue, any easier for the op to bear or worthy of other people's derision. When you read a thread and there are many people on it saying they've been there, many people acknowledging the real and mitigating factors in the op's own life which have meant she experiences the world the way she does, you accept that perhaps this is one area where you have no experience and nothing to offer.
Yes people when they get pregnant know they have a 50-50 chance of boy or girl. The occurrence of gender disappointment as they call it, is nothing to do with the logical, rational side of your brain. It blindsides people. It's like me wading onto a phobia thread and saying you do KNOW don't you that a piddly spider can't harm you at all? It is not a normal reaction and it's beyond the control of the person suffering.
There are people out there who would give their right arm to have a baby? Well yes of course there are. The op isn't one of those women or maybe she is. It's irrelevant. She has her own set of feelings. She is entitled to them. MN is big enough for everybody. You tell her to count herself lucky but who says she doesn't? A woman has room for more than one sentiment. She can know she is lucky to have a healthy baby but still be utterly at the mercy of a reaction she doesn't particularly want. As I said up thread, this sort of gender disappointment is surprisingly common in women with fertility problems, particularly with ivf. Women who have desperately wanted a baby, any baby and suddenly out of the blue this utter, consuming desperate disappointment attaches itself to the sex of the baby. It is nothing to do with rationality. I have said this time and again.
It is also worth noting that this is also a manifestation of antenatal depression for a lot of women. Even if you can't understand the disappointment in isolation, if you can understand depression then you should find it easier to not make disparaging, unhelpful comments filtered through your own pain. Unless you would tell a woman with postnatal depression to get the fuck over themselves, at least they have a baby, it's unfair to treat a pregnant, hormonal woman this way. I get it. I do. People have their own very real disappointments and they might be lifelong and debilitating. Stomping all over somebody's own troubles is not going to help either party though.
It would be lovely if MN was big enough (in both senses of the word) and supportive enough for everybody. I understand that AIBU doesn't allow for such a sentiment but perhaps we can acknowledge that the op chose the wrong topic and allow her an honest reaction and help her through it. There is nothing to gain from attacking her.