How long ago did you have your children?
I remember reading something somewhere about a link between orgasm and fertility, something on the lines of you are more fertile when you orgasm, so it was something like natures way of preventing you becoming pregnant too soon again.
It may well be load of twaddle, but I do remember reading it!
I do think sometimes that when you have young children your attention can be elsewhere (unconciously) and that can make it harder to relax and be 100% involved. I have been guilty of doing all sorts in my head, whilst my body was elsewhere!
At least you have been able to have an orgasm, many don't ever, my MIL never had and couldn't understand why we had sex if we didn't want children(in the days when we were living together, and used to sleep together at her home, she said she thought it very odd that we got pleasure from it!)
My GP says that your body is also starting to change around 38-40ish, it is the beginning of the peri-menopause, the 7-10 year run up to menopause. I can rememeber having a lot of menstrual probs at that age and that he told me that sexual dysfunction is a very common complaint amongst this age group
This won't help matters for you, but my DH says I suffer from premature orgasm! Five minutes and it's all over! I do think that sex does get shorter as you get older, unless you make time to have a good session. I know we have to try and wait until our two teens are aout for the evening before we can get down to anything on the sofa instead of in bed!