I can't believe being monogamous isn't natural. If you can have a life long friendship with someone as many people do, you can have that with someone you love. It has it's moments when it does need more work, compromise, understanding etc.. but any relationship worth saving does - even a friendship. I have friends that need to know the truth and only being a true friend can do it, same with a marriage or life long partner.
If you've been burned in the past that moulds your future ideas of people in certain situations, I've certainly displayed certain behaviours just because of an outcome of a relationship. But, I guess that's just me, I made the choice to not fall for and have kids with a bum!!! (How Judge Judy of me!)