Well, first off, when it's purely casual, guys don't tend to have much in the way of standards. As long as she's vaguely good looking, she'd probably get the D.
What differentiates a girlfriend material girl from a causal hookup girl, at least in my eyes, is simply the chemistry. The personality, how well you get along with each other, how much you like being around each other, how much they accept you for who you are.
I mean, I'm talking to a girl now (it's not official official yet, but I'd say there's a damn good chance of something coming out of it), is she the best looking girl I've seen? I won't lie, I have seen more visually appealing women (though to me, she's by no means bad-looking), but the chemistry is there, we like each other for who we are and we get along really well. I mean, on our first date all we pretty much did was walk around the nearby city, but it was quite easily the best date either of us has had purely because we were with each other. Looks fade over time while personality is who you are on the inside.
You also need to consider that people's idea of beauty is purely subjective. Someone who you might consider a 9/10 might only be a 6 for someone else. Some people think girls who try to make themselves look like Kylie Jenner are hot af. I think they're bland, unoriginal and not very attractive at all.