I...had the pleasure of hearing a Libertarian Party speaker earlier this year. Before he even arrived, the room was abuzz about this chap. You see, he is known the county over for speaking to high school seniors and using bestiality - yes bestiality - to illustrate libertarianism. "You can have sex with a horse and, so long as you don't infringe on anyone's rights, you can go ahead and do it." Never mind that animals can't give consent, but the fact is that's the worst possible way to spread your ideology...period. Instantly, you lose prospective voters who might otherwise agree with you.Libertarianism can be summed up in the following statement: "You are free to do whatever you want so long as you don't reduce other people's freedom to do whatever they want."
Here is an argument that some (maybe most) libertarians use to promote bestiality. According to them, libertarianism holds that pursuit of happiness is moral. Sex in pursuit of happiness is selfish and also moral. Sex between humans and non-human animals is also moral under libertarianism.
Sex between humans and non-human animals or any trans-species intercourse has the benefit of zero risk of pregnancy, meaning there is no worries about having to rush to the abortion clinic the day after.
Some people criticize bestiality, saying that non-human animals cannot consent to sex and therefore sex with animals goes against their freedom. This may be true, but if we uphold this idea then killing animals for food would be immoral as well and we should all be vegetarians. When humans kill animals for food the animals certainly don't consent to being killed. Killing an animals is done at the expense of the animal's will and for the pleasure of meat-eating humans.
When you kill an animal for food, the animal almost certain doesn't consent to being killed because by the laws of evolution all animals try to live and not die.
Animals gain pleasure from sex because sex as a behavior is favored for in human evolution. Most animals gain pleasure from sex although many may not enjoy it if they do not consent.
The bottom line is that when an animal is killed there is almost a zero percent probability that the animal consented to being killed. However, when an animal is the victim of sexual intercourse, there is a positive probability that it consents and enjoys the sex.
Therefore, based on expected probabilities alone, raping an animal is likely to do more to increase the animal's welfare than killing it for food.
If pleasure if the basis of morality (e.g. if we adopted a utilitarian system of morality) then if killing animals is moral then it follows that raping animals is also equally moral if not more moral because the animal victim of rape is more likely to enjoy being raped than being killed.