I certainly don't think that being a wife or a mother is inherently unsexy or desexualizing - but that is the way that it is still often viewed in our culture - and some of that may be internalized by the woman. There's still a huge amount of discomfort with mothers displaying their sexual sides and this can creep into interpersonal relationships. If your man never relates to you as anything other than his partner and the mother of his children and doesn't relate to you as just a woman, research demonstrates that it can be very hard on her libido. Sex researchers refer specifically to the desexualizing nature of those roles - I'm just repeating what they say - the point being that they shouldn't be, but too often they are.
I've never had that problem in my marriage either, but lots of people do. Here's something further that I wrote a while back about how it shouldn't be the case.
