Sex and marriage are two different animals that may, but do not have to, be bedfellows. Marriage is an institution, a social contract for manging property, services, risk sharing etc. - and as any other contract it requires a certain nontrivial level of good faith. If one of the clauses of the contract stipulates no sexual liaisons outside the conjugal unit - it is like having exclusives for a record company or a publisher. You better keep that part of the contract or face the consequences. Of course, nothing forces you to consent to that particular clause - you may remove it by mutual agreement with the other party - but secretly breaching it is plain and simple act of chating on your business partner.
Why people sign the exclusive right clause when signing a marriage contract is another thing . I do not thing that most make a fully informed, rational decision here; just like they buy a car or a house or using a credit card - they do it for instant gratification disregarding lond term consequences. But if they are unhappy with that clause later - that itself implies they made a poor judgment.
As far as sex is concerned - it certainly does not need marriage anymore, thanks to the progress in reproductive health. But marriage does lower the transaction cost of sexual relationships quite considerably, hence its continuing appeal as a venue for sexual liaisons.
Wojtek