Hmmmm....if they are attracted to both, equally, that's not a choice. That's there "dark matter" of what drives human sexuality choices. IMO, something along the pathway from the standard sexual reproduction trait on the Earthly evolution chart for all fauna and insect life reproduction requires the female fertilization of a male. In an environment of constant life vs death, the traits that survive will follow a pattern.
The point being, the traits are passed along. In such a species, that can only be done one way. After that, who gives shit what two consenting adults...or more, want to do in the privacy of their own little American castles?