@Iluviaporos - Well, the thing is, the Ancient Greeks and Romans weren't oppressed by the ancestors of the people wearing the costumes.
Some might still argue that it's stereotyping and it basically is, not to mention completely historically inaccurate and would probably be offensive to Ancient Greeks or Romans. I don't know if it would be offensive to modern people from Italy or Greece these days or not.
But they aren't an oppressed people so I don't think it's a big deal. It's definitely not on the same level as wearing a Native American costume. There are many levels of racism and appropriation going on there, not least the fact that this is a living culture whose people are discriminated against for wearing the same clothes that others use as a costume. And the items worn are often patterned after sacred clothes that aren't supposed to be used lightly.
As party themes go, I can think of much worse than a toga party.