The following is my response to your main question:
Well, I might not be the best person to answer since I'm bipolar II and have experienced up to hypomania. But in my experience, I may not have been completely aware of what was going on, but I was also aware that there's something a bit off about me. The thing is, though, even if you're aware, it doesn't really occur for you to want to stop what your manic self is doing. I do remember when I thought I was going to become a NY Times columnist the next day and in the back of my mind knowing that this was a bit odd....but I didn't care; I just went along with knowing that I will become Maureen Dowd the following day.
Outside of things like that, the changes in your affect, attitude, etc. (the 'little stuff" may not be in your radar while you're going through your episode. My boyfriend recounted to me one time how I had changed in little things like the way I acted during my episode, but until he had told me, I never knew I had changed that much.
As for the rest, I don't think I have the answer/advice for that. I'm sorry.
I agree that your partner should see a psychiatrist. It sounds like has some serious symptoms and he should potentially be on medication. It is possible that even if he does not have bipolar disorder, he may have a different mental disorder that would need treatment.
To answer the main question: I don't think there is a correlation between memory loss and mania.
It is, however, possible for any person to not be able to remember a traumatic event or period of life because of something called "repression", where a person unconsciously pushes memories into the back of their mind and are unable to recall them. This is extremely unlikely to have happened for an entire 6 months though.
Only a doctor/psychiatrist can say that a person is Bipolar.
Do I remember details of my manic episodes? Yes. I wish I didn't as they're very sad and embarrassing. A manic episode doesn't continue for 6 months to my knowledge. The mania just gets worse and worse by the hour until you've done something so lunatic-ish that the police are escorting you to the nearest PSYCH ER.
Unfortunately, the word "BIPOLAR" is slang, so you can't just do a google on it. Keeping up with this "bipolar connect" forum, you can get good information about the illness from sufferrers.
If you don't have too much invested in this relationship, I would say to drop the relationship, rather than let him drag you down into his problems. Or at least tell him to get in to a psychiatrist before you resume the relationship.
I've mainly just answered the title of your question, not the guts of the description. As far as the sex thing, I just say "to each his/her own"