[MacGuffin]

Honestly, I have found people are going to believe what they want to believe, and it's impossible to change that. There will always be people who think bisexuality is a "fad" for everyone who is bisexual, or that they are "greedy" (like your mom said, or that "it's just a phase you are going through" (like my mom said), or that they are wrong or immoral. You can't change them, and honestly if they don't accept you they aren't worth trying to change in the first place.

What you can do is accept yourself for what and who you are, and not only be okay with it, but be proud of it. And alongside that, you can find the people who accept and support you for who you are and encourage you to be all you can be. Those people are the ones worth sticking around with, and they can provide a great source of strength and support when you are feeling insecure about yourself or your sexuality or anything. I suggest you find your friends who are like that, or else seek out groups and things with people like that so you can meet new ones.