The big myth
Let’s start by clearing up a myth. It is a common assumption that muscles tissues need to be physically stretched (that is, to be made longer) in order to become more flexible. This is not true, your muscles have plenty of length to allow you to do splits right now, it is only your own tension that is preventing you from going all the way down.
I know that you are not just going to take my word for this one – so try the following test. Go stand next to a table and put one leg on it. Depending on how tall you are, your leg should now be at about a 90 degree angle, right? The leg you have on the table is in the position for a side split. Take this leg off the table and put the other one on it. You have just established that you can put both legs individually in a middle split position.
Next, ask yourself this question; what muscles run from one leg through the groin to the other leg? Just in case you do not know much about the human anatomy, the answer is NONE. This means that you should be able to get down in a middle split right now, so why can’t you?