Because dominant omegas can knock alphas unconscious with their pheromones, and when they go through this process, they forget part of what happened. He knows it was a dominant omega who did it, since only they can do that to an alpha, but he doesn't remember who knocked him down. To make matters worse, when he consulted the fortune teller and asked if his partner could be male or female, and she said they had large breasts, he immediately assumed it was a woman – because men generally don't have very large breasts, so he wasn't yet fully considering the possibility of it being a man.

If Grayson is considering Dane a possible fated mate
Then y didn’t he think that Dane could be that dominant omega
Or am I very off the point?