I belong to The Church of Jesus Christ of Latter-day Saints (the largest Mormon sect). Please refer to us as LdS to distinguish us from the FLDS and other splinter groups.
Salt Lake City is only 40% LdS, despite hosting the world-wide Headquarters and largest LdS temple (two separate buildings).
The LdS Church sees all sex outside the bonds of marriage, so if this couple are LdS they were acting outside the teachings of the Church. The LdS also has one of the largest charities in the world, so very few members live on the street.
Not all LdS wear temple garments - just those who have been through the temple. These garments are holy vestments, but not considered magic. Many other sects have clergy where the ministry is their full-time employment. The LdS Church has a lay clergy and open priesthood, so our holy vestments were designed to not get in the way of our 9 to 5 while still serving as a reminder of our covenants.
Finally, the couple was outside a Catholic chapel. Btw, Catholicism is the second largest denomination in Utah.