If your dog is trying to bite (or growl) at you that's definetly an 100% NO to sex!! He doesnt want that from you And feels threatened by your advancements! Stop making any further attempts immediately before you end up in the hospital in need of several 100 stitches and he gets put down because of aggression over your own doing, id imagine growling took place before more aggressive behavior did (9 times out of 10 dogs usually give a warning like a low pitched growl if they want you to stop whatever your doing, its a way they warn a bite is about to happen if you don't back off) and if your not taking no for an answer, he will make his desires clear; or rather lack thereof. The only right answer to what can you do? Respect your dogs decision and move on from the idea that he'll ever be a willing sex partner before you put him and yourself in danger, there is no "training" a dog for sex, especially not one that is responding in aggression! They are either into it or not: pressing the matter any further IS ignoring your dogs free will to decide for himself what he wants and could end very bad! Please, if you love him, listen to what he's expressing to you, stop trying to touch him there!