Here's what happens. There is something to do on a Friday or Saturday evening, something fun and grownup that will last late into the night. You and your husband both want to go, but you cannot both go. You cannot leave the baby with a babysitter for that long, that late at night - it would be a cruelty to both baby and babysitter. So one of you must stay home with the baby.
So now you must decide what is to be done. It is an absurdity for you to go and your husband to stay home, as these are invariably things that he has gone and found out about, things that involve his friends or his interests more than yours. It is similarly nonsensical for both of you to stay at home - why do that if one could go? But if he goes and you stay - and this always seems to make the most sense - you will feel miserable, lonely, and defeated. You will weep churlishly - why must it make the most sense for you to miss out? You will forget to enjoy your time with the baby, who, blissfully oblivious of your despair, grins and coos and babbles, making you feel even worse.
That's it. That's the big problem; indeed, that, along with its variations, is the only big problem. If you don't have a baby yet and think you have a modern, highly-evolved relationship, you probably find this to be really silly and imagine that there are many sensible, reasonable, and fair solutions to satisfy all parties. And I am telling you that it is not silly, and that there are no sensible, reasonable, and fair solutions to satisfy all parties.
If this posting seems ungenerous or spastic or whiny, that's because it is; I am writing in a fit of pique, and I have had entirely too much coffee today. I will probably regret posting this, but even if I do, I will not take it down, because it's the truth. This is what happens, and it will happen to you.