As a supporter of white nationalism I don't view interracial relationships favorably, but I don't believe in banning them - at least not directly.
Whats destroying the white population is immigration and programs intended to "integrate" the races.
I do think the white nationalist movement is a little sexist as you usually hear stronger condemnation of females crossbreeding then males although I consider them equally damaging to our goals.
To prevent the destruction of the white population I support a number of policies such as stopping immigration, removing programs promoting "integration" and for Europe pursuing a voluntary repatriation program for recent immigrants(which make up the bulk of the nonwhite population).
oh, and I agree with Rubystars completely in her posts in this thread, modern liberalism promotes interracial relationships and attacks anyone who doesn't dream of living in a "color blind" world where race ceases to exist.
edit:
Eastern Europe although not as genetically "clean"(for lack of a better term) as Western Europe is also far more "racist", I consider Eastern Europe/Western Russia the two places the white population will have the best chance of existing if policies do not change.