Bi the Way: Actually, pretty common. While the original distribution of sexualities along the Kinsey scale (for the eldrae) would have been pretty close to that found in humans, the brain modifications done under the general category of “xenophobia elimination” seem to have compressed the scale at both ends a little as a side-effect, leading to something resembling this trope although still slanted statistically to one side (since the original curve wasn’t exactly a bell curve).
Not a side-effect that anyone was terribly bothered about, inasmuch as local attitudes tend much more towards “exclusive sexuality is a statistical curiosity, but…” or “ah, you Terrans and your quaint little categories” rather than “your sexual orientation is a DEFINING FACTOR OF YOUR ENTIRE LIFE”. And, hey, more freedom of choice!
(See also Everyone Is Bi for where further individual modification tends to go, sexuality-wise – further down the timeline.)