I couldn't do "FWB" because I would inevitably develop romantic feelings for him, and also would want him to be exclusive to me. Not just because I'm jealous, but because I don't want to sleep with someone who has multiple partners without knowing for sure if they've been using protection.

Kudos to you for being able to have sex without romantic attachments involved and keeping the relationship with a more casual and open feel. It's something not a lot of folks can do.