Your fiancé’s knows about your financial situation and current career trajectory as you get into your marriage.
What's the point for them and you to get into an unhappy marriage over this from the get go?
If they value other aspects you bring to the marriage and enjoy a good chemistry should be expected that they only be encouraging and comfortable with the idea of you possibly earning less.
Otherwise if that is what your fiancé’s really wants - someone who earns just as much he/she should acknowledge that need and find someone else than marrying you with that sword hanging over your heads for the rest of your marriage.