Yea, the current term for your beliefs and actions is called "serial monogamy"
"When I am in a relationship" is very telling. Or, do you also mean that when you're not in a relationship you'd never have sex with more then one person even if you've not committed to one another quite yet?
Adding link: (I'm not saying the link pertains to your particular dating strategies/beliefs/dynamics, Insomnia... just adding something I find interesting. There are many other article that have a different slan on the definition of "serial monogamists" t then the one below.