Pre-pandemic, CX did not typically release 2F at booking on 6F 777s. The days of 9F 747s are behind us, alas.
CX isn’t even flying F to any USA city at the moment. Just A350s. The schedules right now are completely fictional. Given the fleet and how HK was turbulent before COVID, I don’t think it is anything like a lock that BOS will be served with an F plane. It wouldn’t shock me to see the US lose F like Australia did some years ago.
I think it’s a little crazy to book yourselves on different cabins on a honeymoon. Which one of you is going to get left behind in J getting the not as nice treatment? In your circumstances I would book 2J and see if F is available for both of you last minute. Otherwise, enjoy your honeymoon and married life together, in lie flat business class...