Your current relationship aside, (because the previous poster has valid questions and you may have reason to be acting this way with this particular man) you do this because you don't trust the men, the strength or the longevity/stability of your relationships.

That's why you're always prepared with a back up plan.

People who feel secure with their partners invest their time and effort into the present and if discussed, the future together.
They don't formulate contingency plans for if and when the relationship ends.

How to stop?
Learn how to cultivate a healthy level of trust within a relationship, then you will know how to "relationship" properly.