Let's look at this. According to "Americans for Divorce Reform", 40 (maybe even 50) percent of marriages (we're talking hetero, now) will end in divorce, if current trends continue (some interesting break-downs of age groups, etc., can be found here). 40%? So much for the sanctity of marriage. Sounds really fucking sacred, doesn't it? I can see why you want to protect it.
Face it, in the end, it's all about a bunch of overly-conservative types who cannot relax and accept that people actually love and enjoy each other's company. It's not that there's any real reason not to allow this, it's they they don't want it because they don't like it, and they want to tell you what to do. Your marriage is not going to be any less solid just because anyone else gets to be married...nor it is gonna be any MORE solid if they can't.
I don't give a fuck who you choose to fuck, as long as they are adult and consenting. Your bedroom activities aren't hurting me one little bit, nor is it going to screw up my children to know that you love each other enough to want to stay together for life...but then, I'm not one that wants to brainwash them into thinking only one way, unlike some others.