Over time in marriage, couples get into the routine of being married and things no longer feel as fresh and new as before.
Kids, increased responsibilities, more tasks, etc, these things take their toll and tend to dampen the energy of 'bedroom activities.'
At that stage, couples need to become extra careful, and become very deliberate in ensuring that the marriage continues to be as good as it was, because if care is not taken, sex will reduce to the barest minimum and eventually die off completely.
Different sleep schedules
If couples are not in the bedroom at the same time, how are they supposed to have sex?
Understandably, work and other activities might make this impossible some times, but couples need to understand the importance of intimacy in the marriage and work a way around this in the best manner possible for them.
You no longer care about your looks
When one of the partners no longer pays attention to his or her looks, it could be a pointer to an impending problem.
No matter how comfortable and relaxed one feels in the knowledge that they are married, it is not an excuse to disregard looks and appearance
Yes, your partner should not look out for anyone else to meet their sexual needs, but you have to try and not make the idea repulsive to them.
Foreplay gets worse with time
If sex has always been about getting straight to the point and ending it as soon as possible, then it might become just too boring to keep trying.
Even in instances where foreplay was once a thing, but it keeps reducing, it could be dangerous for your sex life.
Communication dies down
All the issues listed above can actually be corrected with effective communication.
If, however, communication dies down too, then there might be no more remedy.
So whether your sex life with your spouse is dead already, it is still active and you hope to keep it that way, then avoiding these things should be your priroty.