Pre Marriage it would have been a toss up between my first threesome and the afternoon I found out that my girlfriend was a submissive who would do nearly anything. The two involve different people and happened several years apart but both of them led to a lot of shenanigans.
Post Marriage it's a tie between my wife managing to accidentally self-discover she liked anal (a truly funny and hot story that does not involve a dick) and a few days later when she purposely tried DP and found out that it gives her a rolling orgasm.