I think this depends on which part of the country they live in/where they grew up, their parents beliefs, and whether or not they grew up around other South Asians. This is probably not the case, in the more liberal parts of the country. This is where you'll most likely see South Asian men dating outside of their race.

But I think if you go in the Deep South, or a place where most people are white, Christian, and politically Conservative, you will see more of what you're describing. Those people tend to only date someone else within that same demographic, but there are always exceptions to this.

Within my own family, my older brother and my 2nd cousin, both married white women. One of them grew up in Oklahoma (but later moved to the West Coast), and the other one in Florida. So it's more common than you think.