That's not true, in fact many folks who have majored in other areas pursue MArch degrees... roughly 50% in a given program is usually made up of non-arch background individuals. Many programs want a diverse body of students with various backgrounds. Where did you hear otherwise? That being said, you may need to take some summer courses in areas such as physics and calculus if you did not have those as part of your undergrad curriculum. Some programs may also require architectural history classes, but again that's nothing you can't take in a summer session beforehand.

You should talk to some of the faculty/staff/students at RISD and see what kinds of students applied to their program in order to get a feel what you need to do to get your application together.

You do NOT have to get another bachelor's degree.

For architecture students holding 4 year degrees, there's the 2 year M.Arch.  For those in ALL other fields, there's the 3+ year M.Arch.

They will love your undergraduate degree and they will love the fact that you went to RISD.  Just make sure that you convey in your portoflio, through your designs of apparel and other creative endeavors, that you are capable of morphing into the 3 dimensional realm of building design.

They say 3+ year M.Archs. are for ALL majors.  The reality is that they are more open to people with backgrounds like yours.  There was no one stellar in my 3 year class, but an industrial design major in the 3 year class behind mine was very talented. 

I doubt it is 50% but certainly you can pursue the M.Arch with your degree from RISD.  The two best resources are 

http://www.naab.org - 

http://www.archschools.org - 

I think there are about 50-60 programs that you could possibly attend.

many schools I visited said they admit about half non-architecture majors into their MArch 1 programs.

The whole nomenclature around M.Archs. is so confusing.  Some people mean the M.Arch. 1 to be the first professional degree aimed at those who do NOT have a background in architecture, and the M.Arch. 1 WITH advanced standing to be the 2 year program that absorbs BAs and BSs in architecture.

It's easier to denominate them by time (years) required as some have done in the past, such as M.Archs. Option 2 and Option 3.  That said, in most schools Option 2 is almost always a noticeably larger class than Option 3, often by a 2:1 ratio.  The exceptions would be the Harvards and Columbias where the 3 year programs are a calling card for them ... and for the student.

If someone with an architecture background ends up in a 3 year program, it's generally that their portfolio or performance wasn't that good and/or that the school isn't that rigorous, at least in mandating design competence across the board. 

Learned something from one of the links above (^^):  that Quebec is really its own entity in more ways than one.  I already knew McGill and Universite de Laval (in Quebec City) did not offer the M.Arch. 3 year.  Now, I learned that Universite de Montreal does not either.  Evidently, that province does not like that model.  The places to do this program in Canada are thus quite few.