Sex with the same person forever would be boring?  Actually the oppisite of that statement is true.  Our brain produces a hormone called Oxytocin.  This is a bonding hormone.  We give this off during sex which makes us attach to our partner.  Scientists call this the Super Glue Effect. However, studies show that with each sex partner you have the less and less of this hormone you give off.  If you have multiple sexual partners you have given the benefits of that strong hormone away before you meet your husband/wife.  Therefore the bond did not take place as stongly as it should've and can lead to difficulty in marriage. 

This hormone promotes emotional bonding, increased sexual compatibility, decreased critical thinking abilities, and impaired memories of negative experiences.  This is wonderful for marriage.

We also know that men are visual bonders- they like what they see.  However, if theyare  exposed to multiple sexual images (via sexual partners or pornography) the significance of seeing thier future partner naked diminishes.  Essentially he begins to devalue a womens body.  Unfortuately when he sees his future bride naked for the first time he'll barely notice it.  This means his brain will not help his body bond to her as he should.  Sex will not mean as much to him and he will bore easily with her.  

In fact studies show that the people who are having the best sex are those who have waited and are in a strong monogamous marriage! 

Wow that Rocks!