As a middle-ground voice:
My husband & I are religious. We grew up conservatively. This life was something we NEVER considered until the last 1-2 years. We absolutely view our marriage as sacred, but maybe not in the traditional sense. One of our central ground rules is that secondary relationships are just that: secondary. Family is what comes first. Even just between the 2 of us, our child comes first, so it only makes sense to us. We will not allow a girlfriend (or in the future, possibly, boyfriend) to split us up, just as we don't let family or friends come between us.
But, as Shannanigan alluded to, some people are wired for monogamy. Some aren't. Some aren't but don't realize it, or don't realize they're allowed to feel that way. Some is nature, some is upbringing. I don't believe anyone's wrong unless someone get's hurt.
Just my 2 cents.
Steph: Wife, Bisexual, 24 year old mom to 1
Patrick: Husband, Straight, 23 year old dad to 1
Currently exploring Poly together, our latest adventure.