Schneed10 is most likely more correct. As you get older both your libido's slow down so your not as horny. I'll say I got married at around 28 had my first kid and second 2yrs later. Keep in mind after you have them you have to raise them for 18 yrs. You do the simple math and round up, If your 30 add 20 yrs to it and thats just your first kid. You'll basically be free again around 50 yrs old. If your closer to 40 then your stuck until 60. Average male lives to about 65 y/o. Average female 70.
All I'm saying is everyone I've met has the same story. Whether it's libido, or being too exhausted because of taking care of kids and working it's the same story. Obviously I don't know what I'm talking about. Your marriage is totally different. I hope you keep up the excitement. But for fun.... do your own survey. See what all the married people tell you about their marriages.
I even surveyed the ladies. Same issues. Except they said after being with kids all day changing diapers, feeding, and bathing they were too tired to do anything else. Then they said the male failed to romance them any more. It was all go to work, come home, help the kids with homework, eat dinner, get the kids to bath and bed and then pass out.
Maybe I'm totally wrong about your marriage. Good luck to you.