Disclaimer: I am NOT married, so my experience is limited.
Yes, guys get too tired for sex.
No, I don't think you're asking too much. But you need to consider a few things: How healthy are his life habits? If he's unhealthy, he's going to be less interested in sex. If he's healthy and fit, he may be more energetic and more interested - encourage a healthy lifestyle. Also, when you do have sex, do you initiate it or does he? If he prefers to be the one doing the seduction, you're not going to get anywhere by pushing the issue.
Good luck!
Also, I think it was noted in another thread that you knew how to use the search function even as a newcomer. If you use it, you'll find this thread:
http://www.tfproject.org/tfp/showthread.php?t=123789
And this one:
http://www.tfproject.org/tfp/showthread.php?t=37338
That might help.