“Contrary to popular belief, this is not a compliment.”
If he calls you his “main,” that is not a compliment. What he is essentially saying to you is that you are not the only one. Nine times out of 10, if he calls you his “main,” he has given all the other women he is seeing the same title as well.
In short, “my main” is a polite way of him telling you that he likes you enough to sleep with you but not enough to commit to you.