It sounds like he may not be ready for a relationship yet if he still talks about missing his ex. And the fact that he warned you before you had sex, that he didn’t want anything serious, may be a confirmation for you.

Don’t beat yourself up over it, it happened. Try not to obsess over this either. Just keep in mind that he TOLD you he doesn’t want anything serious aka a relationship. If that’s what you’re seeking, you might need to look elsewhere to avoid getting more hurt. You can see if he replies normally. Just don’t expect a relationship now that you’ve had sex. Not everyone views sex the same way. It’s probably better to let him heal from his ex and for him to choose a relationship when he is ready. Don’t rush him or it will bite you in the butt. You don’t want to be with a guy that still has feelings for his ex girl.