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.