There are other valid sexuality’s for them.
They can use gay and lesbian for example.
Gay means non-women loving non-women, and lesbian meaning non-men loving non-men.
Semibisexual is most commonly used as a straight person trying to be in the LGBTQ+ community. I’ve never encountered someone who used it in the way of being attracted to one gender.
Or another not common sexuality that’s used is monosexual, meaning they’re attracted to one gender.