I think Pax nailed it - its a peer issue with her. She is probably subjected to "anti-gun" noise from her friends, might even join in a bit. From her perspective, how would it look if they found out her husband not only has guns, but carries a gun with a small child in the house!!
If you haven't been married long, these kinds of things (assuming you have a strong marriage) tend to iron themselves out over time. My wife was a little that way too - still is. She doesn't want other parents knowing anything about her husband's guns. And, actually, this is a good policy for many reasons. Over time, however, she's discovered that most of our kids' parents are gun owners, hunters, etc. As long as I keep most of my guns locked up, or concealed from ordinary view (including around the house), she doesn't have any problem. We're happily married, we do lots of things together, but we also respect each other's autonomy when it comes to things like "concealed carry" or things that are specific to her hobby.