There's a lot of moral outrage over what he said, but the problem is, sadly, I don't think it's an unusual opinion at all. Especially among the older gay community, and dare I say it, some of the younger ones too.
I've got a number of older gay friends and they all hold this opinion. They think "you can't have it both ways, you're either gay or straight and that's it". No matter how much the subject is debated they are immovable. We have talked about it over and over during the years, and even though some of them faced horrendous prejudice in the '70s when they came out, they can't see their attitude is just as bad as the bigots who condemned them!
I also work with a 26 gay man and he thinks the same thing. His opinion is that you might experiment when you are young, but sooner or later you will "go one way or the other".
Sadly, I think society hasn't moved on as much as some of us would hope. However, it may be a good thing that the subject has been brought up as more openness, honesty acceptance and understanding is very much needed.