I have been in a casual-sex relationship for a few weeks, and when I see him hanging out with other girls I get jealous. How do I ask him about the girls without sounding like I want more?

Obviously you should not be having casual sex with this person, because it's not casual to you! Pretending to be casual is just turning yourself into a casualty.

Tell the truth, stop faking it: you CARE. The realistic options are (a) tell him how you feel and take your lumps if he doesn't reciprocate or (b) end the relationship.

You're just setting yourself up for pain and possible abuse with this arrangement.

Tell him your casual relationship is over, and he now has two options: a serious relationship, or no relationship.

I am in a relationship like this also. If it upsets you that bad then you really need to either tell him how you feel or end it. Don't make yourself run ragged over this guy. I too often get jealous when I see my guy doing that but I remember what we agreed on. I agree with another answer...If he starts to have sex with someone else then end it...I really don't think you're ready for a casual sex relationship.

You dont and cant do that.

I'm in the situation and you have to remember the agreement that you both had made the first time. Its horrible and makes you feel a bit horrible too, but you have to remember, he's using you for sex, so you do the same and keep having fun and looking elsewhere in the meantime. Use him, like he uses you, and try (although its very hard) to turn a blind eye on these other bints. If he starts sleeping with one, then stop it, as you aren't strong enough to deal with it by the sounds of it...don't worry though, i'm not either. I think that goes for a lot of people in this situation, frankly.

Stay strong, and play the game.

Just chill out, if he gets the idea that this somehow bothers you, he'll probably just leave.

It will be hard to act like you don't "want more" because that's exactly what you DO want.
I think that you should stop this friends-with-benefits thing because you are starting to have feelings, and it doesn't seem like he agrees.
Don't keep setting yourself up for heartache.

If you're in a casual relationship, you shouldn't care about that.