The act of sharing a bed with your child is not, in an of itself, a bad thing. There are numerous, perfectly valid reasons that children end up in bed with their parents. I am a big believer that appropriate, affectionate contact with children is a wonderful way to assure your children that they are loved and cherished. I am also, however, a big believer that there are some behaviors which are limited to your partner and yourself, and it is not the business or responsiblity of a child to take over for an absent parent or spouse when their mother or father needs comfort.
The mother in this situation, by the original poster's description, was citing loneliness and emotional vulnerability as the reasons she wanted her teenage sons to alternate sleeping with her. In effect, she asked them to substitute for the husband who no longer shared her bed, so that she would not have to sleep alone. This is not a mother who sought extra hugs and kisses from her children in appropriate daily settings, but a lonely woman who somehow forgot that her own need for adult, peer physical contact was not something her children should have had to provide for her. The actions of this parent, when examined for their intent, are creepy. Period.
I don't believe anyone should ignore that little voice in their head that says 'something just isn't right about this'. The son, all these years later, is still troubled by the contact with his mother in that situation, and that alone is enough for me to acknowledge that--whether intended or not--the mother's actions were at the very least questionable.
If a divorced dad cited vulnerability and loneliness at bedtime and insisted that his 14 and 16 year old daughters take turns sleeping with him while he spooned them, you can bet your bippy that the girls confession of feeling very uncomfortable about what took place would likely get a much more sympathetic response than "get over it".