I can’t say it’s common, and wouldn’t ever have considered asking some random good looking guy to join us, but straight guy MFM is probably more common than you think. When men on the internet talk about threesomes they seem to be thinking about 2 women, but I’ve only done the other kind and it seems like way more fun - I’m straight so nothing really exciting about adding another woman, but two guys? So many things you can do whether they are into each other or not.
I do think it’s more common for couples to approach women though, as I have been propositioned by couples and probably all women have.
10 is people who want something that will kill them or someone else.
9 is people who want something that would land them in jail, or literal fetishists who get off on something that doesn’t include what most of us think of as sex, and only that.
8 is people who need something other than plain sex or can enjoy things that most of us find too gross
7 is people who identify mostly as their kink but can enjoy sex without it
Which would leave me at 6. I think way more people are in the 1-5 group than the 6-10 group, it’s not an even distribution. If you are asking where I’d land in the general population of women, that would probably be a different answer.