How to avoid marriage discussion in relationship ?

Your first problem is coming right out and saying you're not interested in marriage "right now" because the first thing a women thinks about is, "Okay, but what about ever?" The reason all your girlfriend's have pushed the topic is because they don't want to waste their time with you if you don't ever want to be married and they do. In other words- if you hadn't mentioned it at all, they probably wouldn't have mentioned it for a while yet.