It’s not about being bisexual it’s about being afraid of being vulnerable with the person you’re trying to spend the rest of your life with. If you can’t be fully open and honest with them out of fear you’re not ready to be married, a life long commitment, at least not to that person who scares you. They might reject you? Then they’re not a good match anyway, give them the opportunity to make an informed decision. Don’t have it figured out? Ok, considering it’s not about being bisexual that’s not an issue they should still know you’re at a stage in your life where you don’t have that figured out yet. I think denying a potential life partner any significant information about yourself is not ok, it shows a fear of intimacy and vulnerability and it robs them of the opportunity to make an informed decision, none of those things are good for a life long partner to be dealing with.