Is being gay a lifestyle choice? I thought the mainstream opinion is that choice has quite little to do with it.
Seriously I wouldn't call it a choice. Nobody goes and reads up on hair color and then decides "you know, I think from now on I am not going to prefer blonds, but rather readheads." To reduce it to a choice is just silly.
But the anthropological consensus is that sexual orientation is a cultural construct. These can be binding. For example, I suppose I can choose when spending a $20 bill that it has the same value as a $5 bill but there is no reason when I am being paid to choose that a $5 bill has the same value as a $20. So in this regard, I suppose one may argue that sexual orientation is as real as the difference in value between a $5 bill and a $20 bill....
Outside the USA, yes. Sadly, a majority of Americans thinks that being gay is a choice while being obese is genetic.
As for being gay and American opinion, I'm not sure what this is based on. Polls seem to differ wildly, e.g. compare: http://www.cnn.com/2007/US/06/27/poll.gay/ and http://www.pinknews.co.uk/2009/12/18/poll-suggests-almost-ha.... I suspect the results of these polls have a lot to do with current politics and how questions are formulated.