I assume the Deltan scale would simply be ∞.
Sex is sex no matter who its with. I suspect they, as a culture, would find such discussions on orientation to be either amusing or absurd.
As stated before, I dislike the pheromone theory. In TMP why wasn't her line "I have taken my weekly pheromone inhibitor, Captain"? She would have had no control over them and would need medical intervention to stop everyone from sniffing around her all day. Sulu was a bit enamoured with her, but he may not have met any female Deltans before and simply heard lots of stories and wild rumours about them. Chekov seemed perfectly fine, as did the horn-dog that is James T. Kirk (and lets face it, his libido doesn't exactly need much encouragement).
Instead she needed to take the Oath, so as to protect sexually immature species. I'd take that to mean that sex for Deltans is still a free choice and, that by taking the Oath, they are swearing not to interfere (as it were) with people from other races.
That is how I always took Deltans to be.