Avi K – in the case you are talking about, he was telling him NOT to marry a particular person. Throughout this discussion, I was considering the idea of a Poseik or Gadol telling someone to YES marry someone.
It seems to me that the two are very different. I can hear much more of a svara for a Poseik or Gadol to tell someone NOT to marry someone IF he feels that the potential spouse has specific deficiencies in hashkafa or halacha that will be bad for the person in question.
It is much harder for me to see how someone can tell someone else that they MUST marry someone whom they don’t want to (again I’m not coming to argue against RHS).
In the above case, it is possible that R’ Moshe deliberately refrained from issuing a psak and merely phrased it as advice.