Well normal doesn't necessarily mean good. I understand as we age our testosterone levels decrease and so the sex drive will go down with it, but we are talking about 40+. I read the numbers on generally sexless relationships (complaints are pretty even from what I understand from both men and women), and I've listened to husbands complain about how the sex died off pretty soon after either marriage or a first child, and it's definitely not healthy for either individual and for the relationship.
Urbanized modernity certainly doesn't help, with the constant presence of low to medium levels stressors, and maybe even greater levels from either striving or financial concerns, etc. But sex has a therapeutic aspect to it, especially within a committed relationship. I don't understand why something therapeutic is the first thing to get dropped in many cases.