Why are finances so taboo in a relationship?

Why are finances so taboo in a relationship?
So me and my friends were discussing this the other day. One is a married woman the other one is single. The other one is in a serious relationship.

we all grew up in different financial backgrounds.
It kind of shocked me when my married friends said we didn’t talk about finances until marriage.
I’m always one of those persons that I need to get myself together before being with someone else.
Why are finances so taboo in a relationship?
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