Actually, in all fairness, to play as their advocate, there could be 2 underlying motivations :
1 is of course as you say, selfish attention-seeking, typical of hollywood celebrities.
2 is, for some of them, they genuinely feel that by speaking up, they're giving a voice and helping confused people (eg. some still trapped in religion and feel foolish guilt from their sexual tendencies that don't conform to dogmatic society's or their church's doctrines) to accept themselves simply by adding a voice that they are not alone, they are not some freak that don't deserve to be respected, etc.
The truth is, as always, a mix of both, for some individuals more the 1st reason, for other individuals more then 2nd reason.
Still regardless of their underlying motivation, some good could come out of it for some confused people.
But like you say, if everyone was spiritually evolved, there would be no confusion over LGBT matters, and there would be no need for religion. Evidently, human society on Earth is not there yet.