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I'm not sure how much morals have changed. A good friend of mine is a few years younger than you (63) and has been in quite a few threesomes. Given opportunity, I'm sure he'll do it again.
If it's not your thing and you'd rather not date someone who's gone there, that's your choice. But saying that this is an indication of how mores have changed... well, to support that, you'd need to be a lot older than you are. Or did things change so much in 6 years?