In MC-15525, Grum indicates that Mojang made the decision to set java.net.preferIPv4Stack to true (java.net.preferIPv6Addresses is false by default). According to the Java documentation, setting java.net.preferIPv4Stack to true completely prohibits the use of IPv6. This decision will have to be reversed (or at least an IPv6 non-disabled client needs to be made available) in order to fix this issue (I hereby confirm it still exists in 1.7.9—java.net.SocketException: protocol family unavailable—, and that the presence of IPv6 internet, which I have, is irrelevant).
I can confirm that it affects Macs.