But whether or not his parents were married is irrelevant, because the law in 1961 was that BOTH parents had to be US nationals (at least) to confer birthright citizenship.
If he was born outside of the US (which I think is very, very unlikely), then he's not eligible to be President. If he was born in Hawaii (or another state), he is.
In either case, since BHO Sr. was never a US national (and could not have been legally married to S. Ann Dunham under any circumstances), his parents' marital status is irrelevant.
Thanks for clearing that up.