Ok, about dogs
1st-It is not legal in all states in the U.S. of A. to have sex with animals, but it is not specifically addressed in all states. In Wash. if the animal is not bothered by it, it's not illegal.
2nd-Dogs do not respond to touch as much as people, so a source of odor is necessary. A male dog will respond better to female odors and smells. I guess to be fair I should have said fragrance.
3rd-Several months ago a man was delivered to the emergency room at a Seattle hospital (dead) after having a stallion perform up his rectum. Where it was not illegal. (Several Years Ago) In Florida a woman was granted a divorce after her husband had their Afghan use her in front of some friends. They had done this privately, but she was not happy with the audience.
I'm not sure you could get a dog to start with a man (gayboy) but after training with a woman you could probably get him to try a male. With dogs everything has to be fun, they do not work, only play. Training is not hard to do, but some knowledge is necessary.
Last-You would need a tall dog. Great Dane, Afghan, German Shephard. I'd be careful of Dobermans and Rottwielers
Humans have the largest penis for their relative size of all animals.
So you may be disappointed after all the work.
Good Luck, It really can be done.