Sex should be the benefit of a good relationship - not the focal point or 'end all'. 

What happens if one of you becomes ill and can no longer participate in sex?  Or an accident?  Does 'no sex' equate to 'no relationship'?  If you can no longer perform though no fault of your own is that justification for your spouse to discard you and look for another sex partner? 

Be careful of such inqueries.  Consider carefully what really makes for a good marriage.  Things like trust, honesty, devotion, committment, etc.  Those are your foundation that builds for something beyond sex.  Again sex should be the benefit of a good relationship.