First, there is a jealosy issue which is there at all ages. It doesn't stop at 14, 16, 18, or even 20, 25, 35... Parents are inevitably jealos of any partner of their kids. This isn't even about incest; it is a normal reaction given the closeness which parents and children share.

Then there is the issue of genetic interest. Ultimately, parents benefit from their children giving them grandchildren. But they won't benefit equally from any son- or daughter-in-law. They, again inevitably, will try to select them, or influence as best they can their children's selections. But although some of that selection may be based on more or less objective criteria, some of it is very subjective, such as if the in-law matches the parental values or social expectations. Not necessarily an issue with girls, but I suspect this is a big factor in boys considered as adult BL's YFs. Of course, regardless of any rational opinion on homosexuality, parents know a homosexual child greatly reduces their chances of grandchildren.

And although the issue of control may not be conscious, I find it nearly dishonest that most people doesn't even consider it. It is even one of the oh so stereotypical conflicts with in-laws. Whenever you have a life partner, you listen to the life partner at least a little above the parents at least sometimes. You may not consciously seek to control how your daughter thinks; but you can't seriously deny that if she has a life partner she will drift some away from you and toward him in how she thinks. This is natural and obvious too.

I would finally say that even if parental opinion against sex were right, it still doesn't follow that law is the proper instrument to pursue it. I think doing pot would be bad for my kids, and I think there are many parents who would agree with me (albeit the number may be decreasing) but I very certainly think that law is not the proper instrument to pursue this perfectly legitimate goal.