The reasons are fairly evident:
1. The focus changing from being a couple to being a family ? if you're lucky ? or the focus going almost exclusively to the kids, if you're not lucky
2. Time issues
3. Resources stretched
4. Biology drives new mothers to focus on baby while Dad is left out; that can become habitual.
I knew someone years ago that said his wife "Went from being a babe to having a babe." Another person told me that kids bring you joy but not happiness.
Since many of us want kids, what can we do to mitigate the pitfalls and maximize the joy?
Minimize these behaviors:
1. Spending most of your time as a family
2. Hovering or helicopter parenting (this does not actually teach kids to be resilient)
3. Parental focus on kid(s) and little focus on the couple unit. In psychobabble we call this triangulation: you don't join as a couple and you both focus on your children (when they're young, they give a lot of love back, whereas you might need to work on your couple relationship. It's becomes easier to give up on your spouse and get love from your kids.)
Maximize these behaviors:
1. Scheduling and actually going on weekly dates as a couple
2. Going away for weekends without your kids
3. Taking vacations without your kids (yes, they will survive)
4. Making "quiet time" at home where your kids don't interrupt your couple time
5. Keeping your sex life active (get your kids out of your bedroom)
Please read Child.ol'.a.try for the sake of your marriage and to actually help your kids. If you are happier, they will be, too.