Single 19 years it was me and dd for most of them too op.
I agree with many pps she is being offensive and patronising.
I had this a lot from certain people too when I was younger, (split from ex in early 30's) especially my mother (who is in a deeply abusive relationship!) but also a few friends etc who seemed incapable of understanding I was perfectly happy for the most part (I say the most part as I do have mh issues but it's NOTHING to do with being single, they go back to childhood)
I am not a nun, I have had dates and fwb arrangements when they have suited me but as a pp said it would have to be someone bloody incredible to make me give up my independence.
Some I tuned out, some (the worst offenders as it were) I had stern words with and told them to pack it the fuck in!
One particularly heated conversation was had with my mother as I was approaching my 40th and she was all "you won't get a man now" and "you're gonna be very lonely when you're old" because she was in her 60's by this point and she has a very miserable life with dad, she's barely allowed to move without permission! Fuck that!
I pointed out that I'd much rather be single than in her position!
There are many ways to be happy that don't involve being in a committed relationship or monogamous or whatever
Everyone is different
I'd speak to her again but be firm and assertive and tell her it's unacceptable for her to comment and criticise your choice just because it wouldn't be hers, that you don't criticise her choice just because it wouldn't be yours and she should as your friend be respectful and not bug you about this.
If it still doesn't get through tell her to fucking pack it in!
I don't believe it's "natural" at all to be in a long monogamous relationship, there's loads of evidence to the contrary. Much of what we THINK to be true about marriage and relationships comes from ancient, controlling men! Who were not faithful! As a means of controlling women's sexuality, primarily so they could feel they had certainty about whether the children she bore were genetically his and that his wealth was only passed on to his blood.
It's ridiculously outdated!
How many marriages last 30/40/50 years and are genuinely still happy? Not many! I certainly know very few, yes there are more that remain married but I know more unhappy marriages of that length than happy ones, people fear change, plus social indoctrination and people currently who will have been married 30 years plus will still to some extent have been raised with societal stigma attached to divorcing and the women in many cases are financially disadvantaged too.
I think we will see in future generations people being married for shorter and shorter periods of time as the stigma of divorce diminishes
Plus of course cohabiting couples are more likely to split and aren't included in stats looking at how long marriages last.
I've just tried to find combined stats on how many relationships - married and cohabiting - last 30 years plus and can't find anything if anyone else on thread can that'd be interesting.
Given your likely ages op how long has she been married? Statistically I think it's fairly likely she will end up single at some point.