In my opinion, if their kids are teens they should keep an eye out and say something if they think it's a bad relationship but not force the teen out of the relationship unless its for something serious. If their kids are adults and taking care of themselves the parents can make comments and say their piece, but not make demands or restrictions on the children's relationships unless it is something serious (abuse, mistreatment, makes them very unhappy, threatening, dating a syco)
If you really like/love someone & have been dating & thinking about getting serious I always tell my children "Well it's time to meet the family". They may not be receptive at first but it's a start (especially in the marriage role). As for dating, always put pay phone money in your shoe for an emergency in case you need your parents. Hopefully your parents have brought you up to be responsible, mature & have taught you not to get in an uncompromising position.