LGBTs are increasing in number because society has become more tolerant... even fully accepting of them. So a lot more individuals are coming out. "Fence-sitters" are also getting swayed easier so that if in the past, fence-sitters ultimately decide to go straight because of fear of societal repercussions, today, fence-sitters see that the general atmosphere is favorable to them making the commitment to go all-out gay.
Personally, I think more important than the increasing number of LBGTs is that the age of ***ual awareness is dropping, so that we see younger and younger gays. I've seen a four year old boy openly sashaying in front of hi dad with no problem whatsoever... and the dad is even happy because his gay son can easily belt out Celine Dion songs.
When I was in high school (fine, in the mid 80s), we knew that there were 4 or 5 "soft guys" in our batch. They don't hang out together (having their own respective circles) and tried to be toned down in their actions. Today, it's so common to see a whole flock of ten or more gay grade-schoolers roaming around (malls, streets, school, you name it) and flaunting their ***ual orientation with no regard to what other people think.
As I said, society is probably just more accepting today.