I just found out my bf has an account on Manhunt :(?

Honestly I suggest you talk to him and give him a chance to explain. It may be old or maybe he thought you wouldn't get mad. Talk to him, ask him about him and if you don't feel comfortable with him having that account, ask him to please deactivate it. There is no reason for you to end the relationship or anything. If he truly loves you, he'll understand your feelings.