Lying By Omission

[Blowingrasberries]

I’ve just had a row with my partner over his whereabouts at the start of the weekend. We have a stable relationship and have been together 3 years. Prior, he was with his ex for 26 years and they share several adult children. I came on the scene 5 years after they split up.
His ex has made it clear she does not accept me in his life. The rest of his family have been extremely welcoming, but there are times when I can’t attend family functions when the ex is present as she bans me. This is not very often and everyone feels bad about it. If possible the children will put on two events so I can attend on one occasion and the ex on the other.
I don’t live with my partner. There have been evenings where I know he has gone over to his ex so they can have a family get together playing board games etc. He never tells me openly when this happens, I usually stumble across it later. Or he has taken his ex food shopping as she doesn’t drive. It does irritate me and I have said so but I’ve not put ultimatums down or kicked off - I’ve just said it makes me feel a bit like “the other woman”.
Today we had a row because I discovered that he had spent an evening at his ex’s with the family as he had lost their beloved family pet. I would have absolutely NO issue with this at all!! But instead of telling me he tried to hide it. He first implied (by being vague) that he couldn’t come and see me as the family were coming to comfort him. Later he tripped up and said he was at his ex’s but she went out. He then tripped up again and it was clear she had been there (as she should be).
I am really cross about this. By omitting being transparent it makes me lose trust in what he says. He tried to defend himself by saying:

• He didn’t actually lie, he just didn’t tell me everything
• I should ask if I want to know anything
• The reason he didn’t tell me was because he was saving my feelings

I told him my issue wasn’t where he had been etc - my issue was how he had lied by omission and how that erodes trust between us. This isn’t the first time he’s omitted telling me the full story on stuff. I can’t work out what his motivation is. I don’t like that he does it and I don’t like how, when challenged, he tries to assume little responsibility and acts like it’s my job to dig out the full picture, and that motivation is altruistic. I just don’t buy that. Would you?

This is almost exactly what I was going to post. He's not emotionally divorced for whatever reason. You find him out now and then, I guess that there's a lot more that you don't know.

@Blowingrasberries I couldn't work out how to quote your message for a response so hopefully you will see this.

Yes indeed it is wearing, isn't it?! He initiated the divorce. We'd know each other as friends for a few years prior to his divorce. I am his first relationship post divorce. It all seems very amicable and I'm pretty sure she knows about me. He lost both of his parents at a pretty young age and kind of adopted her parents as his also. They all sound like nice people.
He has a habit of imagining how people are going to react to a situation, or how a certain situation will play out, how traffic and never gives it a chance. This is quite frequent and drives me a bit mad. I don't know if it's him imagining how it wouldn't go well or the fact that his EXW is a family psychologist and thinks it's the right thing to never introduce anyone else into their dynamic. I do know they both have a hard time coming to grips that they're kids are adults now. Both kids live at home with her, absolutely everything paid for(including a new car for them every 2 years), kids have 0 responsibilities and it's going to remain that way for years by the sound of it. She grew up quite privileged, he grew up regular working class. He does very well for himself now as exFIL gifted him his business many years ago.
I did meet his Daughter and her BF briefly last winter. Have never met his Son.
His ex doesn't have a new partner. She's not dated since their divorce 3 years ago.
I honestly think they keep these constant family get togethers and holidays for their sakes, maybe marginally for the kids. They are constant, usually once a week. To be fair, he did try one time to have the kids at his for a holiday on their own but they wanted to spend time with their cousins so he went there. And I spent Christmas alone. This is one of the scenarios about his 'it happened once so it'll happen every time' so I know he's not asked since. It's sad that we rarely see his Sister, her DH and their kids. They are so fun and fabulous. But he reserves all of the time devoted to the exes family. We were at his Sister's this summer and she asked to see a recent photo of his kids. They haven't seen the kids for 6 or 7 years! And they only live 1.5 hours away. I'd no idea it'd been that long. Their marital home was in a 7 house cul de sac. Her parents live across the street from their marital home, her 2 Sisters live next door to the parents with their DH's. So it's as if once they relocated to the area 20 years ago and moved across the road from parents, her family were the only people they ever saw. So it sounds quite dysfunctional to me.
I could go on for hours. It's so frustrating and I give less and less of a shit every day. He really is an incredibly nice and loving guy who otherwise treats me very well. it's the only reason I've stayed thus far.