For romantic groupings involving more than one person, consider the internal one-to-one relationships between the parties and take an average. A MFM triad with internal F/M and F/M pairings would be composed of opposite-sex pairings; if it also contained an M/M pairing it would be mixed opposite and same sex pairings.

For some questions this would work, but I would consider asking some questions related to polyamory, since that is a huge part of some people's sexual orientation (and there's a related question that may touch on a different question of appropriation--is polyamorous fic mostly being written by polyamorous people? I personally doubt it).

Anyway, there's a HUGE difference for me conceptually as a reader and a poly person between a triad and two F/M couples or whatever.