Monosexual refers to a person who is romantically and/or sexually attracted to people of one sex or gender only.
A monosexual person may identify as heterosexual, homosexual, asexual, demisexual, pansexual, or any other gender identity while still only feeling attraction towards the one sex or gender. (It’s not about how they identify, it’s about the type of person they feel attracted to.)
This term is most often used to show an opposite of bisexual, pansexual, polysexual, and other terms referencing attraction to multiple genders or gender identities.
