sexuality is something that isn't definite. it can change over time or it might not at all. in this situation maybe asexuality was something he previously felt but after encountering certain people he maybe diverted his feelings to be a bit more "undecided" & not ecactly "conclusive". plus if you think about it he never actually had s** & if he did she has no reason to get mad. shes getting married, to someone she CHOSE to. the only thing she should be concerned about CURRENTLY is makin sure to arrange that avenue & get a wedding dress or whateva shenanigans u do when u get engaged..
her adding unnecessary input & makin him feel bad for the feelings he cant exactly control is if anything, more shi*** & overall worse to do. not to mention that not everyone is an extrovert and confident enough to confess, so that factor should not be blamed on the person. like ffs, introverts exist yknow 😭..