Relationship - am I being unreasonable?

[Rain123456789]

My partner of 15 years (we are not married or have children) chats with strangers online (only women) on Scrabble Go or FB messenger. The fact that they are random women makes me feel very uncomfortable. It is not the first time we have arguments about it. In the past, he cut off an online relationship when I asked him and he confessed that he resented me for it.
He is very sociable, has many lady friends that he meets up very often (college, work, concerts friends) & I am fine with that. It is only those random people... all ladies of course! Am I being unreasonable?

Of course you're not being unreasonable.

What sort of online relationship was it?!

The relationship he gave up for me started in a Facebook private music group that I was not a member of. I realised they were chatting because of the notifications showing on his iPhone. I am not sure there was any flirting (I never read the messages to be honest) but she was always sending him private messages on Facebook messenger and the amount was in my eyes excessive. I had a gut feeling it was not a healthy relationship.

The most recent chat I have witness was on Scrabble go. He is glued to this game. If we watch tv, he is on it simultaneously for example. He told me that he doesn’t really connect to people on this app but enjoys collecting “badges”.
Now, it appears that he has been talking to 4 women on this app. 3 of them are married, 2 are abroad. He told me I could read his chats if I wanted to but he could be editing/deleting stuff as he goes along. He told the woman on the chat I saw: about his recent birthday, where we were, details of his stressful work etc... it might be a start to a more emotionally based chat. I don’t feel good about it.
I would like to add that I only have a handful of real life friends and only one is male (ex colleague - 15 years older than me) so the all situation is pretty much one sided.

You met him in similar circumstances so I'm not surprised you're worried.
He is putting a lot of time and effort into superficial friendships and neglecting your time together.
No can say he's definitely up to no good but I'm with you, I wouldn't like it either.
What was he like at the beginning at the relationship compared to now? 15 years is along time, do you feel he could be flirting?

I met him on a friendship website (not Facebook or gaming). We have a good relationship, I would consider him as my best friend and are very open about our lives. That’s where the problem maybe is: he is cagey about those chats.
I am not sure if he is flirting but I don’t understand the need to have more friends. He is good at keeping in touch with his friend and would meet them on a regular basis (COVID made that harder I guess). I would say 80% of his real life friends are female.