I think it was Skeptic who made a comparision to a married man talking to a single woman. Could it lead to something? Perhaps, but just talking, the danger level is low. Now, let's put the two in an environment that is more dangerous, say taking a Shiatsu massage class together. Is something more likely to happen now that there is physical contact between and probably a lot less clothing? Yes.
gcc2k, that is an excellent illustration. Just as there are interactions between men and women that are completely safe and those that pose a greater risk, there are different levels of yoga instruction.
If a person's viewpoint and class are geared towards exercise and relaxation only, there is no religious influence. If more is mixed into the class, such as a religious element, then there is greater religious influence.
It is similar to marshal arts. Some are just exercise, some are just fighting techniques and others tie in with Eastern philosphies like being in harmony with the Universe. The level of religious influence depends on the class and the instructor.
But in the setting of a yoga class, it's possible and likely that the instructor is there to do more than help you stretch, to help you on your "journey".
This does not match the experiences that my friends have. Perhaps your experiences are different?
Please note that anything can be tied into a religious influence: a co-worker of mine was a statistician, and his view of God, etc. was directly impacted by the laws of probability. He used the laws of statistics to show why there is no conflict with both theism and evolution being true. He would have made a good cult leader; he was spooky to discuss religion with.
So be cautious about what can entrap you into false worship, because anything can. Life is to be lived and enjoyed; it is too precious and short to kill the quality of it worrying about what is and isn't false worship. Surely a loving God would understand that.