Orientations aren't always purely black or white. Sometimes, there's a little bit of white in the black/black in the white and it's up to you to decide if it's still black/white enough to call black/white or if you wanna call it gray. Either option is fine on an individual basis.

If you're straight & feel attraction to 1 person of your own gender, you decide if that means being straight with 1 exception or bi.

If you're gay/lesbian & feel attraction to 1 person of another gender, you decide if that means being gay/lesbian with 1 exception or bi.

If you're aro/ace & feel romantic/sexual attraction to 1 person, you decide if that means being aro/ace with 1 exception or gray-aro/gray-ace.

Same logic applies to gender too. You feel like a different gender once, you decide if you're cis with 1 exception or trans.

You decide where to draw the line.
Maybe on(c)e changes everything. Maybe on(c)e is an exception, but 2 (times) change your label.
Maybe you use a different label altogether or you create your own label or you use an umbrella term or microlabels or maybe you don't use any labels.