It means to get something that you have been wanting or craving. I am pretty sure that this was originally used to describe the long or much wanted “fix” of drugs. However, I now hear this used in many different contexts. Several years ago (before I had a daughter) I used to say this of spending time with other people’s children. “Now that I’ve had my baby fix, I can wait until later to have my own.”
I have heard it said and used it in reference to chocolate—but I must admit, that I have never heard a man say this. Actually, I have heard it used in reference to cravings of all kinds of food.
I have also heard this used to describe having had enough of something, even if the original craving was unlikely. As in, “I’ve had my fix of screaming babies on this flight.”