Roni,
First, let's take a step back and take the generalization that someone with bipolar is not to be trusted out of your question and discuss the individual, who is your husband and the situation you present.
This kind of situation happens all the time and has no borders of race, age, economic status, religion, etc., etc. Hypersexuality is a symptom of bipolar disorder, which can exacerbate behaviors related to "pursuing" sexual relationships. Though this does not mean your husband is experiencing this symptom.
Everyone is responsible for themselves and their behavior. Your husband is responsible for managing his disorder and then is responsible to address any symptoms, through medication, therapy, or coping skills. Maybe he is not as adept as others and has not learned to manage his symptoms. If this is so, he then needs to learn to identify them and take the appropriate action before behaviors become out of control.
You be the judge of whether this was illness related or not. If it is, then he must learn skills to manage his symptoms and learn when he must seek professional help. If he is already adept or not experiencing symptoms, then he is responsible.
Trust is an issue you and your husband are going to have to work on.
Thanks for all the replies, a lot has happened since I posted that question. I do trust my husband but not this disorder. Since April my husband has been seeing a doctor and been diagnosed bipolar II, he's been taken the prescribed meds and seeing the dr once or twice a month since April. He is learning to recognize the triggers and symptoms of his disorder. He seems to be able to communicate better his feelings and that seems to really be helping. I really do believe him when he says he doesn't know why he did what he did, I've learned a lot about this disorder in the past few months from reading, searching the internet and the support of an online spouses group. I think as long as he continues to see the dr and continues with his treatment plan we'll make it!!