Hi,
There are many reasons for different sexual orientations.Being bisexual is his own choice. No blame on that if he did not get married to a woman & strong enough in his decision to continue as per his wish.
Sex is main thing when comes to marriage.As we all know, marriage is the licensed sex between couples.If what you've mentioned is happening between newly married couple, then it should be addressed at the earliest.
Next Steps
Making him aware of what he's doing is not ethical -(not right after marriage) the first step.
Health Tips
Relationship counselling will help the couple to decide how to move forward. Hypnotherapist like me or any other therapist can help to find out the deep rooted cause for this & help him for further changes for a better marital relationship if he truly wishes to change himself.
Take the steps at the earliest. I wish her all the best!