If I'm out with a friend, doesn't even matter if it's my SO or not, it's fucking rude not to pay attention to the person I'm with. I don't even like it when the odd male friend stops listening to me because his focus is ogling some girl 30 ft away.
It's one thing to stop and say to a friend or SO, 'wow that girls is hot, take a look', but don't eyefuck somebody like I'm not there. If I go out with somebody, my attention is with them.
No different than taking unimportant calls or texting when you're out with somebody...bad fucking manners.