I posted a few years ago about my DP of 6 years relationship with one of our colleagues (we work together).
Basically, he was a bit fixated with a younger female colleague and it actually deeply affected how I felt about him (and still does but to a much lesser extent).
He maintains that it was innocent and he was just trying to help her out. I tried lots of times to get him to see it from my perspective but he was fairly resolute in his assertions that I was jealous and paranoid. I asked him how he would feel of a very attractive younger man joined our team and I went out of my way to ‘help him out’. He said he wouldn’t be bothered, as he wasn’t jealous Or crazy like I am.
To give you an example, we had booked a restaurant for dinner one night and he had to go to a meeting in another city about an hour away. Of course it was necessary for the colleague to go with him. Needless to say, he showed up two hours late because he’d taken the colleague for a drink to thank her for her help. 
The situation only resolved when the colleague left and he lost contact.
We got through it and now a new male colleague has started. Objectively, this colleague could be considered conventionally attractive. I could see the dawning of a bit of horror on DP’s face when introduced to our new colleague last week. He has now become a bit fixated on the new colleague’s appearance and keeps asking me if I think he is attractive.
Is it childish and unreasonable of me to be happy that finally he might know how I felt for two years when he was flirting?
Obviously, in reality I have no intention of flirting or saying anything inappropriate to the new colleague, as I wouldn’t want them to feel uncomfortable. Also, I won’t say anything about my views on the colleagues appearance to my DP, as I know how bad this feels.
I’m happy to be told that I’m being immature or unreasonable but this is bringing it all back for me and I’m starting to reconsider our relationship again. Clearly he doesn’t trust me!