My boyfriend wants to have an open relationship but I do not.
Why does he want to have an open relationship? Is he unhappy with your relationship?
He is not unhappy, he just believes in a polyamory lifestyle.
Well i hear its popular now.  I've heard there can be benefits, we aren't really meant to be monogamous...its not in our genes.
Correct, they say that even most animals are not monogamous. But I don't like the idea of having relationships with more than one person.
I think it could be fun.  I watched a documentary about it just the other day where the couple felt true love for each other but kept it exciting because of their 'open' relationship.  I can see pros and cons.
I worry that romantic feelings for the other person would develop. 
I guess its a possibility.  I think i'm ok with the 'idea' of it, if my husband was actually with another women I think it would actually be very difficult to be ok with it.
Exactly. That's the worst part of it. Maybe I should just end my relationship now.
Talk it over, maybe give it a try and see what happens, you might be surprised!