If think the hormonal response and desire to procreate can be so strong in a woman that they would absolutely consider "breaking" their family unit, because that is what they're being biologically driven to do.
I have 2 sons, I desperately wanted 3 children regardless of sex. But I knew in my heart of hearts that as a full time professional, my time would be split to the detriment of my existing children.
DH was certain that he didn't want a 3rd but i had to allow myself to come to terms with his decision and I asked him to let me come to the realization myself rather than him taking the decision out of my hands by getting a vasectomy.
The problem is that I know full well that we had agreed at least 2 children and hormonally I would probably threaten the same as your wife had my husband decided that he wanted to stop at 1.
I don't know what to tell you though, the want to procreate is such a strong and hormonally driven desire that a lot of the time a person will go to extreme lengths to do it.
I can only speak as someone who has happily taken the time to accept that I won't be having a 3rd and actually there is nobody missing from our family so I can totally see where both of you are coming from.
What are your reasons for not wanting a second? Is it that you had a tough experience with your first putting you off or you a second really stretch you financially? The financial aspect would probably stop me in my tracks (or at least delay them) far more than lack of sleep. Humans (and most certainly mothers) are driven to deal with feeding/sleep and lack thereof with a new baby so it may be that she is dismissing your concerns in this regard, especially if she's the one doing the majority of the night wakings.
I don't think it's selfish in the slightest to not want a sibling for your first child, but I do see why she is striking out with the threats. She's trying to protect her biologically driven urge.