Not really. Nothing really controversial about it, it's done in animals all the time. There are religious issues about it, but just like with any aspect of sexuality.
Some religions. Most of the world faiths don't have a problem with this, but some religious leaders try to make a problem out of this for their own reasons. You're free to ignore them; if your religion prohibits it but it's the right thing for you, just don't tell them. I'm all for people doing what they wish if they don't hurt anybody.