Our midwife likened it to if your partner suddenly brought home another wife/girlfriend. How would you take it?
Actually, I really, really hate this analogy. How many women walk away when their OH is unfaithful? If DH brought home a "new wife" the marriage is over, simple as that. How many children can walk away when they are given a sibling? So completely different power play going on. Also, it isn't sudden. Your child has a long time to process it IF you allow that to happen by talking during pregnancy. Heck, I'm not yet TTC and DH and I talk to Liebling about siblings, when appropriate: he has recently figured out that he will always be the older sibling if Mamma has a baby.

My main beef with a "new wife" wouldn't be an extra person in the house. I'd welcome help with bills and housework. I just don't want anyone else having DH's children, which means DH can't be unfaithful. As I am not going to have sex with my children I really, really fail to see the analogy.