I would say not "all couples" have sex problems in their relationship. However it is not uncommon for couples to have problems.
The problems can range from normal health issues, a woman going through menopause, a man with an enlarged prostate, one or both people no longer feeling attracted to their mate or they have gone a long time without having sex.
"It's easier to maintain a fire then it is to reignite a spark" It's a mistake to put sex on the back burner in a relationship or marriage.
Sex is a physical expression of love.
Generally speaking whenever there is a lack of sex in a relationship there is usually very little wet kissing, hugging, hand holding, snuggling, taking showers together or other physical intimacy/touching. Romance, flirting , and sexual innuendo have also most likey been abandoned as well.
"Monogamy becomes boring when couples become lazy."