Sex in a Relationship?

Is sex really important in a relationship? Especially in a girlfriend, boyfriend relationship?

My brother came up to me and wanted me to help him in his assignment? He is freshman in college, so being his sister i said yes. But when he started talking about his assignment, i was quite surprised about his question. Its not that i dont want to answer the question. We are living in the Philippines which is actually composed of christians particularly catholics and according to the churchs' teaching "SEX" is only for those married couple and it is a big NO NO to those not yet married. My brother anyway is enrolled in a catholic school so my answer would be in contradiction to it.

I am a believer of god and i do have a strong faith but when it comes to the issue of SEX im really open minded about it. Things do change..... Im not against it but for me it comes with greaT responsibility on the couple. Will you still believe that couple in a 4 year relationship did not experienced having sex? Maybe but maybe its unbelievable, except if both really can control the urges which is very hard.

Back to my brother, when he asked me that question well the only answer i had was, IT DEPENDS.

Is SEx really important in a relationship or should it be a part of it? I hope anyone can share their thoughts on this........Thanks