Well he's up to something and it's one of two things:
1) As you said, he may be trying to downgraded your relationship to just a friendship. Although, when you say you guys have sex from "time to time" it suggests to me that this thing is pretty casual anyway.
or
2) He is trying to make you jealous in hopes of converting this thing into a more solid boyfriend/girlfriend relationship.
Either way it's good that you are noticing this behavior he's exhibiting as he's definitely up to something.
Or he's trying to hint at a 3 way. But I still agree with MM on this one.
Time to have a nice "chat" with the boy.
Gosh you should have laid down the law wayyy before you started having sex with this guy. Because to him, he's already thrown his cards down. Casual sex = fuck buddy.
It's not to say that he may not want more, but in most cases when a guy becomes that flippant about mentioning other women while in your company...We're talking going past the point of no return here...He down right catapulted your behind into the friend zone X__X... but remember...most "friends" aren't seen as sexual entities.. But its a guy, and they'll pork anything just to say they did .
I know that sounds insensitive, and I'm sorry :(. We've all been there. You obviously care about how he's perceived this relationship. You can either continue having sex with a guy whose never going to view you as more then a friend(?). Or, you can slam the breaks on this before it gets wayyy out of hand.
This has happened to me before and it could be something else... It's possible he thinks that YOU want it to be more serious and he is trying to let you know he doesn't want anything more serious by talking about other girls. Start talking about other guys.... fight fire with fire