so maybe you should try to be in a relationship? staying in state like this risks your friendship already imo, so I think you should try after all, and even if relationship wouldn't work out there are chances that you would be still friends.
Maybe he didn't ask you on a date because you said you're okay with being friends? 'but I made it clear to him that I would be okay with that' this seems to me like closing doors for something more, just friends and kissing, nothing more...
'One night, we were cuddling and fine. The next, he's bringing over some girl. It's ridiculous.' that was cruel of him, but as I said, if he agreed for terms of friendship with you, he can have relationships like this with other girls too... I know that it's very very hard, but you need to ask him straight about his feelings, the other girl, where is a line in your friendship and what 'friendship' means to him (for me if he kisses other girls not only you it's not a good sign about what kind of person he really is :c)... I know it's hard to risk something nice, friendship which helps, etc, but if you won't make choices, someone other will make them for you... Sincere talk is the best thing you can do now imo for yourself and for him, till you wont talk emotions will grow...
good luck with everything Girl, and don't forget that even if this friendship won't last, you will find someone other in future hope for the best, prepare for the worst