Agree. If it were three out of six couples, or something like that, it would be one thing. Six out of seven invited is hurtful.
I was friends with a girl all through school and young adulthood. Bridesmaid at her first wedding, let her live with me when she left the waster a few years later, helped her get into and furnish her flat. When teens we were frequently overnight at one another's homes (though I think her parents always thought my family was lower-class than them, and were aloof) and she referred to my father as her "second dad."
She met a surgeon when we were around age 30 and pursued him like a big game hunter in Kenya. Finally bagged him with pressure from his sisters. Again I (who was perenially single) helped her plan the wedding, served as bridesmaid, my parents decorated the chapel, blah blah. I thought we were best friends for life.
About a year into her marriage she said something like "Bill and I are going to host a formal dinner party; I'm so excited about planning it. Black tie. It's too bad you aren't dating anyone as we're only inviting couples."
Mind blown. I never excluded her from anything when she was single, or would have dreamed of doing so. We stayed in touch for a few years but I never really felt that same after that and withdrew from the friendship; just stopped responding to cards/e-mails etc. That was more than 20 years ago. Ran into her sister a couple of years ago and after some questioning, told her that anecdote. She looked aghast and said "I'm embarrassed to be related to her."
People can have weird notions. The older I get the less likely I am to tolerate their bullshit.