The author here makes some interesting points:

Regardless of the warnings or information, there are always Catholics who say they will not give up their yoga because it makes them feel good and they personally don’t use it for religion. But there is a further consideration with yoga. By participating in yoga, or when a school or church sponsors classes, it gives the message of blanket approval. If yoga is okay with the Church or with Sally Stretchy, then it’s obviously okay, is the impression.

I think part of the controversy is a communication issue. Some people use the word yoga and mean stretching. Others use the word yoga and mean the religion. If we all used the term with the same meaning, our discourse might change. If we all meant stretching, then it is not always a sin for a Christian to practice yoga (stretching). If we all meant the eastern religion, then it is always a sin for a Christian to practice yoga (another religion).

Since the word does double duty in English, I don’t ever plan to tell someone “I’m doing yoga later” or anything like that. Because even if I only mean “I’m doing stretching later,” the person I’m talking to may not know that the word has two meanings. If the other person decides to sign up for a yoga class to get started, those classes often blend yoga spirituality in with the stretching, so now the other person has begun practicing another religion in part because of what I said.