Yes hypersexuality can be a part of bipolar but there are many other symptoms as well. If you are concerned her sexual fantasies are unhealthy best to discuss that with her in person and with a relationship therapist and she could see a talk therapist on her own. There is also a seperate forum to discuss those issues. If she has bipolar there would definitely be a lot more going on in her life so it might be good to think of what it is. If she is diagnosed with bipolar her psychiatrist should make a referral to a talk therapist (psychologist) to discuss these specific issues. What you describe could just be roleplay which is consenting adult. If its a fixed obsession of hers to an unhealthy extent then that's something that should be talked over with though. If you are afraid she will engage in this behavior with someone else, she most likely won't have an extra marital affair but is seeking this fetish and it all depends on the extent of it as to whether its pathological or not.
Sex remains a mysterious matter along the history of mankind, and this is part of its beauty because man has tried to elaborate on new ways, techniques, etc...
also whether Bp or not, meds or no meds, there are situations in which one of the partners imagines himself/herself that he/she is sleeping with another person in order to raise his/her emotions/urge/desire in order to reach orgasm faster or easier.
Contrary to this, while in intercourse, one very easy way to elongate the duration in order to maximize the pleasure is to think of something completely different, such that the mechanical side is the only present, but of course continues to function using inertia.
Now since your wife is in her highs right now it's the first situation which prevails. As IL said i don't expect her to look for someone else, but only seeking to enjoy the fantasies of sex.
don't worry, you are a cowboy, so you must be able to ride pretty well
Honestly, I think you are doing a wonderful thing for her. It's great that she feels open enough to talk to you and as long as you are satisfying her she won't go anywhere else. I know from experience that having thoughts about someone else can get overwhelming. You don't want to want them and you know you shouldn't but your mind and body don't cooperate in that state. I have personally gone outside my marriage to satisfy that craving, even when my head told me not too. If it doesn't bother you then there is nothing wrong with it. Have fun ;)