"While it clearly isn't considered offensive in the US, it's still a pretty rude gesture"
There are a bunch of other gestures that meet the same criterion. I am Indian, so I'll list some I'm familiar with:
1. Not addressing your teacher/professor as "Sir/Madam". Addressing a teacher as Mr. X/Professor Y is verboten.
2. Displaying photos of your family at work (mild, but yes)
3. "Disrespecting" your elders/teachers/bosses with counterarguments.
...
On the social front, you'd be expected to
1. Not address your elders solely by their first name.
2. Not cohabit with the opposite sex before marriage.
3. Not smoke/drink.
4. Not eat pork/beef.
5. Not change your gender.
Which of these did you object to/are you going to object to? Will you refuse to work with someone trans-gendered, as would happen "instantly" in India?
The fact is, you were in the US not India. I would argue that the onus is on you to adjust yourself to the customs of your hosts rather than the other way around. Just as an Indian would expect any US (or otherwise foreign) visitors to keep in mind the Indian customs.
PS: you used two epithets to describe your teacher in the same paragraph. I find it amusing that your "Indian" sensibility (Guru! Respect!) is not offended by it.