psychiatrist here. this is not medical advice.... i am unaware of any evidence that bipolar disorder is more common in gay vs straight populations. and i would bet against it being true. however, i am also unsure that there have been any studies that have looked at that issue, so lack of evidence does not imply that it's not true.
the main reason it might SEEM that bipolar disorder is more frequent in gays is that many crappy psychiatrists, non-psych MDs, and laypersons confuse bipolar disorder with borderline personality disorder traits. at least half of the people that i see with a diagnosis of bipolar disorder from other MDs are really just borderlines. the reason for the frequent confusion, i think, is that borderlines frequently behave in very hysterical fashions and do things that are outrageous. to an MD or layperson that has not seen true bipolar disorder, a borderline acting up can seem to meet all the criteria for bipolar disorder.
it is very difficult to describe borderline personality disorder in just a paragraph or so. think super drama queen -- pushing people away but wanting them simultaneously, manipulating, feeling lonely and unwanted, needing to test friends and lovers, seeing others as either wonderful or terrible and switching back and forth easily. very rapid swings in mood -- like in a moment or a day. a popular book about borderline that really describes the disorder well is called "I Hate You, Don't Leave Me."
in contrast, bipolar disorder features manic episodes, and often depression, but the switch from mania to depression takes many days, not instantly just because you didnt compliment their new outfit. there many be long periods of normal mood between mania and depression. mania often features vastly increased levels of energy, and i dont mean just being spastic, i mean sleeping 3 hours in 5 days. the borderline might do drastic things during a rage, like boil your pet rabbits or key your car, but the bipolar during mania will do things that honestly won't make sense at all. they may become grandiose and decide they can start a business by selling off all their assets in one day, or buy 5 new Mercedes in one day, etc.
to further confuse things, many medications used for bipolar disorder are also used off label to reduce certain behaviors in borderlines.
unfortunately, many mental health workers disparage borderlines. as we know from dealing with drama in the gay world, very dramatic people can be difficult to deal with. and there is a sense of frustration because borderline personality disorder is very hard to treat.
nonetheless, the disorder should be taken seriously. borderlines have a very high rate of self destructive behaviors. these often seem to be attention seeking -- like "overdosing" on 10 advil and then calling 911 -- but frequently result in severe consequences.