Well first let me say that you will never really understand bipolar. That is great for you but will also be a source of fustration your entire life with this man. I use the analogy of giving birth. I can read every book on the subject, talk to thousands of monthers, know everything there is to know about giving birth, but in the end it will only be the the palest of reflections of what is actually involved in the act of creating life.
You can no more understand what goes on in our brains than we can understand yours. The big problem is that BP presents so differently in every person. Heck even on this board we have such a wide variety of responses to our BP it is mind numbing. Some of us have had sexual issues, others none. Some of us have used drugs and alchol agian others none. Stealing, money issues, racing thoughts, manias, depressions, to name just a few, and many of these things alot of us have never had an issue with. (with me i have never, ever had a desire to drink or do drugs. Now stealing and sex are a completely different subject)
Things you can do, and this is assuming that he gets help and gets properly medicated. (BP gets worse with age, not better) Find out what his stress triggers are and help him avoid them. For me it is money, i stress very bad about it. however my wife does not work well with me on this issue and it really makes it hard not to freak out, which brings on a major mania in my case.
All you can do is love him for who and what he is. I'ts not his fault that he got this no more than if he had a brain tumor. However what is his responsibility is how he handles it. He will need to make sure he talks to a therpast, takes his meds, and keeps the lines of communication open to you every day. Those things will be very hard, but with your support he can do it if he wants to get better he can be better.