I really dont get it, tbh. My reaction would be the same as that guy. "Oh cool! Never seen a lesbian couple before!"
It's not mean, and its not unthinking either. At worst it could be described as curious.
Its not like he yelled "Oh hey lesbians!" And drew a bunch of attention to them. Someone below said dont treat them like circus attractions, and thats not what he did. You are allowed to be interested in other people, and as long as you arent rude about it, who cares?