simcha during Av - the heter of tzorech mitzvah

The Shulchan Aruch writes that once the month of Av starts we minimize business activity, we cannot build for the purpose of simcha (e.g. a wedding hall), or plant a garden for simcha, and we do not schedule weddings during this time. The Rama adds that for mitzvah purposes these prohibitions are lifted. By the Rama’s logic why would there be a problem of having a wedding during Av – isn’t getting married a mitzvah? The Magen Avraham answers that we don’t have weddings because it would be bad mazal. In one of the Mesorah journals R’ Soloveitchik is quoted as giving a different answer which sheds more light on the nature of the issurim here. The Rama does not mean that mitzvah need pushes off the issurei aveilus of the month of Av in "aseh doche lo ta'aseh" fashion. What the Rama means is that the issurim of aveilus only apply to acts of simcha. In order to define an act as simcha-related, we need to take account of that act’s purpose. If someone builds a wedding hall, the building is an act of simcha; if someone builds a beis medrash the act is one of kiyum mitzvah. This applies to building, planting, business activity, where purpose serves to characterize action. However, getting married is by definition an activity of simcha – the fact that an added kiyum mitzvah is also accomplished is irrelevant. Therefore, weddings are prohibited as part of the minhagei aveilus.