@LemonShark
So if he is planning to go meet a female friend I'd probably expect to be invited: I wouldn't necessarily go, but I think it shows respect and that there's nothing to hide. I'd invite him and he often declines but the option is there.
Oooh, yes, good point and well said. That is perfectly reasonable to me. Your whole post is totally reasonable and in fact sounds just like us. Personally, I would probably try to decline or reschedule if DH couldn't also go for whatever reason, as I'd hate for him to miss out or to go have fun without him, but I do totally understand that that aspect of our relationship is a bit weird. :) We're probably that obnoxious couple people say are joined at the hip and roll their eyes, and if you get one of us, you get both of us.
As for the poor OP... I think my head might pop right off in OP's situation, where it's a new mate I've never heard of nor met. Something about it being a new mate makes it worse, as if he were out looking for new mates, perhaps... It's much easier for anybody to accept opposite-sex friends that a DP has always had, and a partner doesn't have the right to force a friendship to end, but a new female mate is eyebrow raising, IMO. I would find it far more than simply inappropriate - in that situation I think I might go all green eyed monster and paranoid!! I'd be angry for sure. And then of course the lying on top of that! And with her being heavily pregnant! He's being a Grade A, First Class shit.
I don't want to even be having to have this conversation with him. I shouldn't have to be in the first place.
@OP you're absolutely right. You should NOT have to have that conversation, particularly right now at 36 weeks, when he should be spending every spare second making certain that you and baby are happy and comfortable and safe and secure and well cared for. He's spending every spare second chatting up some woman you don't know instead and going out of his way for her, and that is completely unacceptable IMO.
I wish you the best of luck when you do speak to him about this. And I know it's awful and sickening, but you MUST, and immediately. Put your foot right down straight on top of his, hard and let him know exactly how you feel about it, that the constant texting and staying up late just to chat to her MUST stop now, that the lying MUST stop and never, ever happen again. Ask him how he would feel - and tell him to actually be honest with himself rather than just answering what he would want you to say - if it were you doing these things with some other man ^and LYING TO HIM about it. Make him put himself in your (too-tight and uncomfortable anymore) shoes and face facts.
In short, NO you are NOT being unreasonable.
Also, if I were you (that is, if you're feeling up to it), I think I would be very tempted to do the dinner, and keep a very close eye on the pair of them when they're together.
Do let us know how you get on.