Me too, op. And it isn't just a "it's summer, everyone is busy and hard to reach" thing.
My partner's dad NEVER invites us over to his many huge family gatherings at his house (yes we invite him but he's always busy, and when he does come once a year he hovers by the front door, he has literally never sat on the sofa). He is remarried and prefers his second family over his first (when he dies his full inheritance is going to his step children, not his bio ones) but also my partner and I are the only ones who never managed to have any children, and he loves kids, so he just doesn't bother with us. My partner's siblings, and all his step siblings all have two or three kids each so no one bothers with us.
We say "we don't care, phew, we avoided THAT!" etc., but it does hurt.
Not really sure why it panned out this way. I was always very social and friendly, but just never get invites. I guess we can be quite introvert but then I wonder if we were just made that way by always being excluded.
I have also never had "The Girls" and find it a really fascinating concept that women have a huge bunch of mates to call up and go out with!
No, we aren't dicks or anything. It's just how it seems to be.