Heteroflexible
From wipipedia.org
Heteroflexible is a neologisim which means a person who is primarily attracted to members of the opposite sex but might have sex with members of the same sex. It is in distinction to bisexual which denotes routine interest in, or participation with, both sexes.
This is perhaps most often found with submissives or slaves, who might be directed by their master or mistress to have sex with someone specific, likely to be another lover of theirs. This may be used as a training technique, or to emphasize that the Master or Dominant has exclusive control of the submissive.