bisexuals are attracted to two or more genders, and they could have a preference or not. "Bisexuality is a complete and fluid identity. Do not assume that bisexuality is binary. In fact, don't assume that there are only two genders." Bisexual Manifest (1990)
they are different sexualities and both are valid. they coexist and don’t erase each other so there’s no reason for invalidation towards any of them. if you still don’t understand please search and be open to learn and get educated.