It’s probably similar to sharing a drink though, or eating off someone’s fork.
I wouldn’t share a drink (bottle) with anyone but DP. Occasionally if we go to the cinema he will offer our shared drink to his brother or his DCs and then I don’t want to drink out of it again, as to me, it’s crossed a line.
I also remember being utterly freaked out when I first met DP’s ex and she walked in to his house while we were eating, picked up his fork and took a bite of his dinner. To her (and him) this was obviously acceptable behaviour - in my mind she pretty much took a territorial piss all over him!
I think context is everything - who is the other person, what is he like about sharing generally - drinks, food etc, is he a regular smoker, is she, were they on their own, were there other cigarettes available or are they both trying to cut down so went for half each, is this a relationship which crosses other boundaries, eg too much texting etc and this feels like another boundary?
My instinct is that it would annoy me but I’m quite intuitive and notice little things that are out of place.