Posting here because of the vote function which could be useful. Otherwise I suppose its a relationships post.
Going to keep it simple. I don't live with DP, but we spend a significant amount of time together, at least 5 days out of 7, if not more. Together on and off 6 years.
The other day his phone rang, female friend who he has not mentioned or (as far as I know) seen, or spoken to for over 3 or 4 years hame flashed up. A long time before we were together dp and said female friend nearly had a one night stand (drunken) but because of the level of alcohol he'd consumed it didnt go ahead. Anyway, flash forward to the night his phone rang, I said aren't you going to answer that? He said "no, I'm busy", (he wasn't!). I asked, have you spoken to friend lately, "yes I bumped into her we said we might meet up", me:"OK, how's she doing? How come u didn't mention it?", DP starts shouting about how doesn't have to tell me everything, why don't I tell him about things (load of gibberish, I share lots of info about my female and male friends with him, id certainly mention if id bumped into someone I'd not see for 5 years)..
Anyway, he later admits they didn't bump into eachother, they text eachother, and it wasn't a vague plan, they had agreed to meet up whilst I was at work, on a day last week, during school pick up times (as she has kids). When I left for work on the day they had (unbeknown to me) agreed to meet up, I said to him (as I always do on my way out) "got any plans today?", and he looked me straight in the eye and said "no nothing special, might go to my mums". Then I went to work to earn the money that pays for us to live, and it now turns out he waited for her to turn up (she didnt). Flash forward to the evening of the phone call, and me finding out about this plan, when I asked why the hell he lied to me about it he said "because she always bails on me"...
Aibu to think that this isn't a good excuse, and he didn't mention it because his intentions with said "friend" are not entirely "friendly"?!
For the record I've never met her, but I know she's very attractive as he's bragged to his friends about having nearly got with her 
Don't get me wrong, I don't think she is interested in him, so I'm not in the least concerned about cheating.. But I'm so angry at him for keeping it from me. I think it's sneaky snakey behaviour, and shows a dubious intent.
Its also disrespectful to her, if she thinks they are meeting up just as friends, yet he keeps it from his girlfriend.. Am I unreasonable to think he's being snake here? I
I have BPD (EUPD) and had a turbulent childhood full of my parents infidelity, and being abandoned, so can over react to perceived betrayal.. Just want some opinions please :).