It's called dating, not marriage. If the 2 sat down & said we're a couple, & we won't be seeing others, or they get engaged, or married it's exclusive & you're cheating. Otherwise go out with as many people as you can. Variety is the spice of life, but if you weren't out sampling how do you really know what you like. I think a lot of divorces are caused by folks looking back & realizing that they never sowed their wild oats, & now feel stuck. I tell my son's to date a different girl every week.
Fritz is right. This happened to me in college. This guy asked me out then asked out my roommate the same week. I just figured he wanted to see which he liked better. Seemed rational to me. After he made his decision, we dated exclusively for almost a year... he never once cheated on me.
Next time he calls, say you've already made plans with someone else then immediately hang up the phone. If he calls back and asks why. Tell him he obviously doesnt have a problem asking other people out, why should you.