This happened to me in the early 00's when I chose to divorce with 2 small children. I was financially independent with my own home and that too isolated me from friends. Older and wiser now. At the time I felt very alone and it took me a good 10 yrs to build up a new and genuine circle of friends that I still have to this day. It was a largely lonely time in terms of friendships and groups. I have great friends now, a mix of mid 20's/30/'s, divorced peers my age and still married peers my age- all met through work over the decades.
What I have learnt is that ( and I know this will grate on some and does not apply to all but .. ) generally , coupled women, don't usually like attractive single/divorced / financially independent working women either with / or without children around! Lots of my divorced female peer friends have all experienced the same so this really is a thing. I heard a few years ago from a friend I had known decades ago when our children were at nursery, who ghosted me when I divorced. She herself is now divorced and wanted to "reestablish the friendship". She told me she "ended " the friendship as she felt threatened at the time as her husband would often comment on my " professional work clothes appearance" and " financial independence". This basically has echoed many conversations that "old friends" have had with me, largely following on from changes now in their own marital circumstance. Needless to say I have not continued with those friendships. I cannot forget that they left me on my own when I needed them the most. Their insecurities , not mine.
This whole even / odd numbers cobblers is nonsense. I have single friends, gay friends, those is insecure or new relationships and long term 25yrs + married friends with grown up children like mine. I regularly host all here at mine. They are my friends and their relationship status is not relevant. Also large family meets. There is no ever " single" person, we are friends, we are family, a community. No one, nor do I , ever feel alone or odd one out. In fact, I am usually always at the centre. I am single ( do not want the boundaries of a relationship) and always invited to parties. I attend with married friends whose husbands either stay home with the kids or their own activities, colleagues and /or single friends. I have even attended a colleagues wedding with my friend and her husband as I had no +1!
Op, if you are being left out or feeling left out, I strongly advise you that you look to broadening your friendship group. It is not ok for friends to leave you out on the sole premise that you are single. It should not define you or your worthiness to be involved.