The truth is, he doesn’t care about you.
If he respected you, he wouldn’t string you along.
If he had feelings for you, he wouldn’t put you in a situation where your needs come second to his whims.
If he saw a future with you, he wouldn’t risk you being available to fall for someone else.
The truth about the guy who wants to “see where it goes” is that he already knows where it’s going to end up. We all know what happens when we don’t put effort into something. It tapers off. It fades. The project isn’t successful. You can’t sign up for a class and expect to learn something if you’re just going to see if you feel like attending it every week. You won’t learn a new skill if you don’t work through the discomfort stage. You won’t be successful at a job if you don’t intend to show up for it. Showing up is what matters in life. It’s what you do when something (or someone) is important to you.
“Seeing where it goes” means he has no interest in seeing it go anywhere. He’s ready to passively sit by and “see” you walk in and out of his life. He’s down for the experience of being with you, because it might be fun for the moment and there is minimal cost to him if it isn’t. He will get to add a notch to his belt (which he has been told as part of male socialization means he is successful in life). He will experience hooking up with someone, and maybe pick up a useful tip to use on his dream girl, whenever he meets her. Every single thing he hopes to gain from whatever relationship he has with you has one thing in common: he benefits. Nowhere in this equation is what may be beneficial for you.
The truth is, this man doesn’t even care about your enthusiastic consent. He won’t tell you the truth about what he sees as a future for the two of you because he knows you wouldn’t agree to what he is actually offering you. Being upfront and communicating with you would jeopardize his chances of sleeping with you. His goal is to experience variety and soothe his ego by getting you into bed. He isn’t looking to invest in you, hear about your day, or even give you an orgasm.
Don’t build an entire fantasy world in the space that vagueness creates. Don’t make excuses for him. The guy who only wants to see where it goes is selfish. He wants a fling and he doesn’t care if you get hurt. This isn’t the way a good human being acts. This isn’t even the way the guy in question would probably act to a friend or a business partner or a stranger off the street. He would understand it’s wrong to treat people as if they are disposable.
There are a million ways to casually date or court someone that respect the dignity of all persons involved. Typically, this requires an overabundance of communication. You can create whatever rules you want as long as everyone understands and agrees to it with full ability to consent. “See where it goes” guy is not doing this. He’s using a cheap line and counting on you not calling his bluff. He isn’t just being laid back and open-minded, he’s refusing to have skin in the game.
The truth is, you deserve better. You are worth more than someone who is lukewarm about the idea of getting to know you. You deserve someone who makes the effort to be vulnerable and be real with you. A relationship isn’t worth having if it’s with someone who isn’t trying to be a good person or take care of you. The absolute worst part about the guy who wants to see where it goes is that he’s taking up space in your life that could be occupied by something better. 
