Hmm, I've been there too ShakyMilk, twice (yes, shame on me). Here's how it happened, which sounds similar to your experience.
A particular name from work keeps cropping up, and OH becomes very defensive/protective of her - with slight variations, in my case:
OH1 - FFS, it's a high-stress environment and she's the only person I've really clicked with, of course there's nothing going on, aren't I allowed to have friends who are women, what's the big deal if she calls me at night, why are you being so needy. (He divorced me and has lived with her ever since.)
[Fast forward several years]
OH2 - FFS, it's a high-stress environment, she's one of the few colleagues I can trust to do a good job, yes she's attractive, what do you want me to say, and what's the big deal if I lend her my scarf, or there's pictures of me spoonfeeding her at the office Xmas do. (He divorced me and remarried sharpish, but not to her as she was in a relationship.)
I finally have a partner who puts me first. After many years together, that's still a wonderful, and qualitatively different feeling than I had with the previous two.
Sorry, didn't meant to hijack your post Shaky with my own story, but just to offer my hard-won wisdom, which amounts to: if it walks like a duck, quacks like a duck...etc.
My other experience, sadly, is that once you're on that juggernaut it's pretty hard to turn around. Date nights, weekends away and marriage counselling didn't cut it for us. They just diminished the divorce pot. But maybe these things can work, in different circs - I have turned into a bit of an Eeyore when it comes to relationships!