I dunno about Bangladesh, I don't even know too much about places like UK or NYC...but the Bengali girls in my community...80% majority are marrying "shada" who they meet and date while in undergrad, grad school, etc. Of course there is a public conversion to Islam and whether or not they really are Muslims is actually none of our business nor our concern.
I would NOT hesitate to predict that more than half of the Bengali Muslims in the US will end up marrying "shada". To each his/her own. Interestingly I'm not sure how many Bengali-other desi marriages there are. I know of 2 friends who both married Pakistanis, I'm scheduled to attend another Bengali-Pakistani wedding in 2 weeks, and my cousin married a Malayali south Indian, but apart from that I don't know too many.
I drop knowledge so heavy it leaves the world unbalanced, exterminating the spiritual force of all that challenge, I'm the lyrical apocalypse that crumbles the granite, replacing you as the dominant species on the planet.