Single women often make people uncomfortable.
Not well adjusted people obviously, but a lot of people are not well adjusted.
They make attached women who are not in good relationships (many) uncomfortable - because you represent a threat to the relationship; someone for their man to cheat with or leave for.
They project their desperation to be in a relationship onto you and presume you must be trying to get a man, any man - too.
Widows and divorcees on here have outlined their common experiences on this front. The corresponding experience to the threatened, uncomfortable, hostile, suspicious attached women .. is the sleazy attached men.
I found they could be sleazy whether they were actually up for cheating/leaving or not. They - again;- project both their desperation to be in a couple and their desperation for sex - onto single women .. and presume that you are available, sex starved and desperate. The cheaters will make overtures. Many of the non cheaters will still have a smarmy attitude in my experience. Some seem to believe that - because you're single - they could have you if they weren't attached. I've many a time thought to myself "no mate, I wouldn't have gone out with you if you were single .... you're delusional".
On top of all the mating game dynamics that make both women and men uncomfortable around single women ... Esp of child bearing age; there is a general attitude that women are supposed to couple up and produce kids and it's unnatural and a waste and a sad case if they don't. So you get lots of "well meaning" questions and low level harassment about why you're not coupled up and what you're doing about finding a man etc etc.
I think some of this is genuinely rooted in the historic situation in our society ... Where women who didn't marry and stayed spinsters had so few options for making decent money and being comfortably off - that ppl were genuinely worried about then remaining single and having no decent/second wage coming in, no kids to look after them in old age (before social welfare, nursing homes etc). Look at how different the perceptions of the word bachelor and spinster are.