I will tell any girl they're pretty. If they're really pretty then I will probably call them beautiful. Or gorgeous or whatever best fits. But I would never tell a girl that they're unattractive physically or ugly, because that's not true. Just because they don't get my heart racing doesn't mean they don't for someone else. I have told a cpl girls that their personality is ugly. Because personality is something people can change. But as far as looks, any guy that calls a girl ugly or not pretty is not a real man in my book. That could scar a girl for life.
This is two different questions. If you a guy doesn't find his own girlfriend beautiful, then why is he dating her. Not all beauty is found in the outward appearance. I've know some gorgeous women that were awfully ugly once I got to know them. And I've known some women that weren't normally on my "attractive" scale, and yet found them to be quite cute and adorable after I got to know them better.
But would I tell a girl she was pretty even if I didn't think she was. Absolutely. Because she may have worked super hard to doll herself up or find the perfect dress, and just because she isn't a girl that I would date, doesn't mean that she hasn't made herself look pretty. All women are beautiful in their own ways.
If the girl is hideous, I would not compliment her as being "pretty. "
However, if she looks okay, or if she is wearing some nice clothes, or gets a new hair-style, then yes, I would tell her she looks good.
"Pretty" can mean anything, really. I think most guys would interpret it as meaning "better than before. " So if an average, or less-than-average girl puts some time into her appearance, I would venture to say that she would be "pretty" compared to her former look.
Look don't get mad at Jarett, all he was trying to do was give his opinion and he seems like a nice guy. That said if a guy is going out with you and says you're beautiful or pretty or stunning or whatever, he sincerely means it. That annoys me so much when I tell a girl they're beautiful and they don't believe me! It doesn't matter whether you think you're pretty or not, because if that guy is with you he will always think you are the most beautiful girl in the world.