Futa is a slur and shouldn't be used.

The word futa or futanari is a term I unfortunately see used against trans women and actual intersex people. It is a term which originated in porn and anime to describe someone with both types of genitals, usually though it just refers to someone who looks feminine and has a penis. So it's no surprise it's very often used to describe non-op trans women primarily. Because it's often used in porn it carries strong connotations of sexualization and dehumanization, it reduces the person being called futa to an object of sexual desire and doesn't respect them as a person.

This term is extremely disrespectful and really has no place being used, especially in reference to trans women. Additionally because of its relation to porn people who use it will almost certainly be seen as chasers.