Well, if the Ramones were punk, then so was Green Day. Maybe they weren't all that compelling at times, and maybe they're wearing eyeliner now (gah!), but punk is punk, even if it isn't your type of punk.
There's no rule saying that punk can and has to only sound one way for all time and eternity... punk came from the people, and as the people change, so will their expression of punk. Punk is as much about singalong choruses as snarling diatribes, songs decrying political viewpoints as much as songs about love or lust. There is no rule saying it can only be recorded in your friends basement to be considered legit and "real punk".
About the only thing I can really say that is punk is the willingness to do something yourself, without catering to the Powers That Be, self-reliance, and self-expression.
If you feel like you have to wear certain clothes or only listen to certain bands to be "punk".... well, whatever, but I think it matters more what's inside a person and the quality of their character than what type of jeans they're wearing or if their music is recorded in a studio or a garage.
EDIT: as was noted, punk can be either mainstream or "underground".... again, the Clash, the Ramones, the Sex Pistols, etc were all very popular and sold lots of records.... that = mainstream. Are they not punk because they sold lots of records?