Yes, kids mark an end to marital bliss, even though they are wonderful. With kids, parents have a lot of things to distract them. They are such a big responsibility, it takes a lot of planning to find time to be together, to exercise, and to consider each other's priorities. Children are a wonderful addition to a family, but it is true that things change after kids.
The stereotype that kids mark the end of marital bliss is wrong. Although it is more difficult for a couple to focus on their own relationship when children require so much focus, it is still important to do so in order to maintain a healthy relationship. Healthy couples are generally happy couples regardless of whether or not children are a factor.