“Lilmod, what is the difference between telling him not to marry a certain girl and to marry a certain girl?Why should he ask a bet din?”
I was assuming that his father felt there was a particular hashkafic or halachic reason why he was doing something wrong by marrying this girl, and therefore he should ask Daas Torah about that particular issue. I didn’t mean a Beis Din – I meant a Gadol hador.
One of the differences between telling someone to not marry someone specific and telling someone to marry someone specific is that there may be a problem with marrying that specific person (which is presumably the reason why someone is telling them not to get married).
Besides for that, it should be obvious that there is a world of a difference between telling someone not to marry someone specific and telling them to marry someone specific. Forcing someone to marry someone they don’t like is very different from forcing them to not marry someone they do.
The truth is that on some level that is done all the time. Anytime that I call someone to ask about a guy who was redt to me and they tell me not to go out with him, that is what they are doing in essence. It has happened that someone told me in very strong terms not to go out with a particular person.
On the other hand, it is pretty rare that anyone told me that I had to marry a particular person (maybe date, but not marry), and the few times it has happened, I ignored them. But if someone tells me NOT to go out with someone, I take it very seriously.