This advice might come as a surprise from a yoga teacher, but if you approach movement with stretching in mind, you are missing a crucial distinction between what you're attempting and what is actually happening.
There's only one thing you need to concern yourself with in a yoga practice - that is, 'pandiculation'.
This is something you naturally do when waking up, and one that animals are seen to be doing all day (we should too).
What is a pandiculation?
It's simple really. Don't think of it as stretching, think of contracting.
Notice what is really happening when you yawn in your bed as you wake up. You are not stretching, but contracting. This is very different to stretching, which only serves to create more tension.
If you are a dancer, yes, you may need more of a range of motion, but most of us do not need it if we do not use it.
It's simple really; if you're weak, get strong, and if you're strong, get mobile. If you don't use it, you probably don't need it beyond a certain range of motion - which you absolutely do need.
However, what earthly benefit can you derive from placing your legs behind your head?!
Mobility is not the same as flexibility, although mobility could be seen to encompass flexibility. In other words, you may be flexible and get your legs behind your head, but you may not even be able to lift one leg off the ground and make a circle with your knee, while standing on one leg in a controlled articulation.
Pulling legs into lotus is just one extreme example of what I mean. Flexible people can pull the legs into the posture, but those with more mobility would be able to do that without using their hands.
Please don't attempt to pull your legs into lotus pose. And while you're at it, please don't attempt to stretch, because that's not what is happening, anyway. Instead, lengthen your body, finding more space to move as you contract and release tense muscles.