It’s different though in that the relationship is different.
It doesn’t make any difference to your family’s relationship with you that you’re bisexual. You’re still their daughter, they don’t have a physical relationship with you, so who you are and aren’t attracted to is irrelevant to them.
However, you have a sexual relationship with a partner, and if that partner only wishes to be with someone else who is heterosexual (which is entirely their right), then they should have that choice early into the relationship.
To tel them years down the line smacks of an agenda and is manipulative.
Think about it. If you enter into a relationship with someone then you might talk about previous partners, previous relationships, and your thoughts and feelings and attractions. Being Bi doesn’t equate to looking for the first opportunity to cheat, and many people are in happy monogamous relationships with people who are bi.
However it’s not unreasonable to think there is likely an agenda if your partner suddenly announces years into your marriage that they’re sexually attracted to other men/women. This is why so many people are saying that coming out well into the marriage is a precursor for having an affair. Because what other reason is there to suddenly announce your sexual attractions after years of marriage other than that you want to pursue something else. It’s naive to think otherwise.
And again, this isn’t comparable to telling your partner you’re bisexual at the outset of your relationship.
It’s not dissimilar to telling your partner years in that you’ve fancied a threesome or to cross-dress or to have anal. Generally people bring up these things because they want to explore them.