Married but having thoughts of other men...?

I've been married to my husband for almost 3 years now. I married him because he was like my best friend and someone I could always turn to. He's a dependable guy. Lately, I find myself questioning our marriage. We're not getting along, there's no sparks in the bedroom, and I can't help but thinking what if I would be happy with someone else. Is it wrong for me to think this? I don't want to have this burden weigh down on my marriage and affect it anymore than it has already. Would it be wrong for me to walk away from my marriage to pursue a life where I may find more happiness elsewhere?