Why don't Space Marines have chem-geld (if it's even available to them I missed it)? Is it already assumed as their humanity so far gone that it's a non-issue?
While I could see automatic, total failure when trying to seduce a Black Templar, I could see someone having success against a Space Wolves. They sure act like they'd still have manly urges (though I haven't read the SW novels, so it may be explained there).
Since some SM fall to Slaanesh, I can't see that it follows that 100% of all attempts to seduce a Space Marine would automactically fail.
On a related note (sorry for the eugenics...but I can't understand why 40K doesn't have eugenics) when someone has proven themselves worthy of being a space marine, they are removed from the gene-pool. This seems like a poor idea. In the grim-dark future of the 41st millennium, wouldn't you run breeding programs so your supply of "pure" aspirants remained flush? Why wouldn't there be a nazi-esque breeding program to assure the "best" people (especially when you're trying to make Space marines)? Why woulnd't you tell your new recruit to run down to the planet and get 15 or so women pregnant (here's a list of who we think will be a good match!) before you ship him and his genetic "perfection" off into space, never to breed again?