The institution of marriage is one not to take lightly. You make a promise to a higher ground, that you will commit to a monogamous relationship.
I fear of my desires to be with somebody else if I ever choose to marry. Not of my actions, as I have full control over them. But of my innate feelings of longing for others, whether it be emotional, sexual or stemming from financial needs. That, my friend, is human nature and uncontrollable. Presumably, a rather unpleasant feeling as it will probably bestow feelings of guilt inside me. Unforsaken amounts of guilt towards my partner, just for having thoughts!