Husband is bipolar, had an affair for a few weeks, my psych said it was probably due to a manic episode with hypersexuality. could this be true?

It could be: In mania, people become hyperactive and impulsive, getting involved in many kinds of activities they might otherwise not do. They can engage in risky financial deals, and promiscuity is certainly possible also. This is not a 'free pass' for hurtful behavior, though. Hoping he's committed to treatment for his bipolar disorder, and that the 2 of you can work in couple's therapy as well.

Maybe: Mania is associated with hypersexuality and impulsive and out of character behaviors. That said - only your husband can say (when he is not manic) if this was a result of his illness or a choice he made.