sex in a relationship is important. It is a way you show the person you love that you love them in that way and no other. It's a bonding experience that is shared only with one another, hence making it special. In not talking about one night stands or quickies, but sex within a relationship. There are different forms of sex so I'm only answering on this particular type.
A sexless marriage typically means the couple have sex less than ten times a year. It works for some and others not. The problem really lies in how much sex each individual in the relationship is comfortable with. If you have two people with a compatable sexual appetite, it doesnt matter how much or how little sex there is so long as each person is satisfied. The problem arises when you have two people with a big difference in sexual appetite. The person who needs more sex feels starved of attention, love, validity, and can begin to turn those feelings in on themselves. They start to blame themselves for not being sexy enough, not lovable enough, not good enough. On the other hand the person with the smaller sexual appetite begins to feel obligated to perform, thus cheapening the act itself and resenting their partner. This is where infedility, seperation, divorce, and other big issues can arise from, if the two dont communicate and try to figure out a comprimise.