It's complicated. I've always suspected that sexuality would be more fluid if we weren't culturally inclined to worry about it so much, but I have no evidence aside from historical anecdotes. I also think that genes are augmenting complicated childhood experiences that lead to sexual development, and that seems to be rather well accepted so i'll go with it.

But the point is that people who are not fluid, who are fairly solidly homosexual, have those attractions so deep in their psyche that you can't change them. Psycho-sexual development happens early. You can't change pedophiles for the same reason, and that one would be a change that everybody could get behind if it were possible. 

Calling it a lifestyle is slightly misleading, since we think of lifestyles as vague life preferences, and sexuality is much more at the core of who were are as a personality than, say, economic class. Sure, a person who is attracted to men could chose to rough it out and act straight, and that had definitely happened plenty of times in the past, but the question that is being decided is this: Why? Because other people are fuckwads? We matured as a society and saw that the real problem was people opposing homosexuality, and that the only leg that group had to stand on was superstition.