Often the second or third orgasm is much weaker than the first. If a spouse loses interest, it could be related to many different things. Maybe they feel too fat, too stressed, etc. Maybe they are carrying resentment towards their spouse. You need to talk to your partner about that.
Counseling is often helpful.
Lack of desire is the number one presenting sexual problem in America. It can be caused by a number of things, such as relationship issues, personal issues, kid issues, financial issues, and betrayal issues. It is very treatable. Many times a couple can find books that are helpful. An interesting test is to see if it is your lifestyle that is killing your sex life.