I see what you're saying, but to get off a horse, you'd step into that left stirrup as you kick the right leg free. This is how you fall over to the left. This is still how i will dismount a stationary bike or a bike on a trainer, though..but I'll also clip in there with left pedal first and stand on it like a stirrup.

I rode horses MUCH more than bikes (20 years riding horses now), but my comfort zone from day 1 on a bike was push off with right foot, put left foot down first. My natural coasting position is right foot in back stroke, so I'm learning to change how I coast so I can unclip left. I am right handed but half the time left-footed (I kick better right, but I'm a leftie for track and field and kickboxing, usually). I don't know if this makes a difference.

I start with my right leg because my left, more stable leg, can push off from the ground better. My balance is pretty poor on the right side for a variety of reasons.

Do whatever makes you feel comfortable and keeps you upright.