I have been wondering this myself,,, several women i know who have great husbands dont like to have sex no more and have little sex drive where as i love sex and want it at least twice a day if not more......and to me this is normal
Men's testosterone and growth hormone levels can drop dramatically as they age starting in their late 30s or early 40s. That leads to a loss in sexual interest.
One good indicator is morning erections. If your man wakes up with them then something else is going on. If he no longer wakes up with them then it's quite possible his testosterone is low (or he has a pending heart problem)
If it is low testosterone, the fix is to raise it to where it was in his 20's through HRT. That will increase his sex drive. Add back exercise and reasonable eating habits, couple it with viagra as necessary or growth hormone (although this is very controversial at the moment) and he should be back in the saddle.