I completely understand where you are coming from.
Here's a site, don't know...others may have already shown it to you, but try it out:
http://www.libchrist.com
It's good, even if you don't agree 100%... it will help you think through some things.
US? We view our marriage as a sacred union of our souls, NOT as a "exclusive sex contract". We 'share' with others that we get to know, sure, but we do so w/in the bounds of our 'together' sex-life. It's simply another fearture of who we are together!
If you have more concerns, PLEASE feel free to E-mail.