Further to your question, I would suggest you speak to a doctor about this. It is not generally a common thing to do, so I think the average person would not be able to provide much more information then myself.
Please don't be offended but to ask for advice on this subject is completely out of the ordinary, I believe you will only receive alot of crude remarks.
I suggest you seek professional counselling or talk to someone about this. I don't understand why you would do this considering you have had several relationships before. Animals don't know any different and even though it may seem like you are not doing any harm, you are harming yourself mentally and maybe physically. In years to come, i'm sure you would not want to look back on this. There must be an underlining reason for you to do this?
I wonder what posses a person to do such a thing, I suggest you get a girlfriend next time.