I told him I needed sexual exclusivity and that I would rather things end if either of us felt like we wanted to pursue things with someone else.

Let me bottom line this for you as a man.

IF you ask for sexual exclusivity BEFORE sex, then that sex better be out of this world good. That sex better be SO GOOD, he decides to shelve all the future sexual interactions he's going to have with any other woman in the future, possibly forever.

And if it's not that good, then he's not even going to bother bringing up a FWB arrangement, because he knows you're not going to agree. Better to tell you that he wants to see other people and cut ties.

You had sex with him on the second week. That's a normal and reasonable time frame to have sex. You didn't fuck him after just having one drink, but even THEN, if the sex is good, the guy won't think you're easy (if he's not a total scum) and will be willing to negotiate a DTR.

Here's how a healthy girl approaches this. You meet, you date, you have sex. You show him the 'goods'. Let him decide if the sex is good enough to give up sex with any other woman. Then you DTR to LTR BF/GF or FWB/Casual. If you want exclusivity before sex, it's like you want guarantees before the event, like you're not confident it'll be good and you want assurances beforehand. That shows a lack of confidence and very unattractive. Stop doing that.