Solidgold, is it REALLY that way? Honestly?? I actually don't think so.
Firstly, to address something that the OP said in the OP. I absolutely think that when you have children, they come first, waaaay way ahead of us, as parents. Thats the key though, as PARENTS, in the plural. Your DS should be paramount to your DH too.
I think it can be very easy for SAHMs to fall into a pattern of doing all the childcare - i am one, and sometimes i do resent it. Sometimes, though, i think my DP rather resents that i CAN do all the childcare.
I have never been a party animal, we ironically had started socialising quite a bit more before DD came along but of course that all stopped when she arrived. For BOTH of us. Yes, i suppose we could have made the effort and gone out either separately or got childcare. But we didn't want to. We have pretty much grown out of that now (we are 38 and 44!).
For me, successful family life is about working together and playing together, pardon the cliche, but it is. I can never understand couples, children or not, who's principal hobbies are separate from each other. I have lots of friends who are like this, the men at the golf course, the women shopping or with their horses. That for me just sucks, whether it is a SAHM or WOHM or childless couple, i just don't see the point of being together if you are not going to be, well, together! Yes, have your own friends and hobbies, but not that take up most of the spare time.
I guess this would be felt even more acutely by a SAHM, as it is very easy to fall out of the loop while the working parent remains within their social circle as it is often linked with work.