It is not an unrealistic boundary. It is definitely doable to not thirst online, nothing more difficult than just scrolling past.

OP is a jerk for crossing it.

That said, it is not a boundary I would keep. Not because it isn't doable, any decent person can do it without coming up with shitty loopholes or diet analogies; I don't keep this boundary with my partner because it would make us both less happy.

My partner and I both just comment on it when we see hot people, but we can only do things like that because we respect the other's boundaries in the first place.