That is not a correct statement. A person who does not have sex outside a marital covenant is not asexual. They most likely choose not to have sex outside of marriage.
It is actually defined both broadly and narrowly, so I think the best thing is to qualify what is meant by chaste. It always means no sexual relations outside of marriage, but is often used to mean celibacy.
However, I see “chaste homosexual” as an oxymoron. Unless you buy into the argument that people are born homosexual, in the same way one is born male or female, homosexual describes an act and a conscious choice, not a state of being.