Yeah, this ... I know three women who live not too far from me, 2 widows and a divorced woman (all in their 50s and 60s.) They've had several married men in the street offering to mow their lawn, put up shelves for them, do the decorating for them, trim their hedges etc etc. All middle aged married men with grown up children who want to be the 'knight in shining armour.' (Why do some some married men do this?)
All three of these women have turned every offer down because of this reason. They didn't want the wives to feel uncomfortable or unhappy. They were all married themselves once, and they know how they would feel if their husband had been tootling around over to single women and widows being their 'knight in shining armour.'
They know how much it jars, and how insulting and disrespectful it is to the wife when a man is desperate to be be seen as a 'knight and shining armour,' helping other women - so he can puff out his big manly chest and feel like a big man helping the poor, fragile, lonely, iccle woman.
Also, although I've not been single since my mid 20s I do remember on some occasions, if ever any man ever came to talk to me (when his wife was not far away,) I would get absolute daggers from her, and evil looks, and even got called a fucking slag behind my back for talking to her husband when HE spoke to me first. I was a 'slag' just for talking to him. 
So yes, some married women are massively threatened by single women/ widows. Of course, not all married women are, but yeah, certainly some are sometimes ... Sometimes they have a good reason to be threatened by them/worried about them, but IMO, it's unhappily married women that are more of a threat to your marriage than single women, or widows.