So long as the sexual activity doesn't affect another unwillingly, doesn't bring a child into the picture in which they are not prepared to care for, and doesn't harm themselves in a way that would hurt those depending on them, then I think couples are free to explore even their strangest of fantasies and the world in the end will still be just fine. Wear animal suits, cover each other in BBQ sauce, reenact a scene from the exorcist, whatever gets your rocks off, it wont make a difference.
And of course as long as they both know exactly what is going to happen. Experimentation can be great for relationships, and doing it with someone you trust is a safe way of experiencing sexual fantasies. As far as I'm aware most people have kinky fantasies; it's normal, and can be fun if both people are into it :) (Also, always use a condom)
First, what is acceptable and unacceptable sexual behavior can be determined by reason and natural law. People, because they have the essence or form of personhood, have a human nature, or an internal principle of change or rest unique to those in the form of personhood. This nature directs the person toward the final cause of personhood, or that for the sake of which personhood exists. What directs people toward the final cause of personhood--and the final cause is happiness--can be objectively determined. Whatever naturally directs people to happiness (the ultimate good), then, is good, and whatever directs them toward a lack of happiness which they ought to have is bad. Perfect happiness is not just an emotion (though it does come with pleasure), but is rather an activity done when the person has fully actualized his or her potential personhood (become most fully the form of personhood, which is the same as having achieved the final cause).
Sexuality is a thing in human nature, meaning it is part of the form of personhood. How a person uses his or her sexuality, then, will affect whether they are actualizing their personhood and acting in accordance with the final cause of personhood (doing the right thing) or not (doing the wrong thing). Thus, any sexual act which goes against human nature (goes against the final cause of personhood) is morally unacceptable. For example, if homosexuality goes against human nature (just as an example--I'm not arguing here that it is or isn't), then it would be wrong to engage in homosexual "sex."
Thus, even if a given sexual activity is performed between consenting adults, it is wrong if it does not serve to actualize a person in accordance with the final cause of the form of personhood.
PS, if you don't understand what I am saying, look of Aristotelian-Thomistic metaphysics before you try to refute me. "It is better to be silent and be thought a fool, than to speak out and remove all doubt."--Abraham Lincoln
Humans are not beasts. It is NOT morally acceptable to satisfy our animalistic desires in any way we want. The most obvious example of that, which I think even the most radical of progressives will agree, is adultery. How can a married person have sexual intercourse with someone else? In such a situation, what kind of respect does he or she show towards his spouse?
'To subdue one's self and return to propriety, is perfect virtue.' (Analects 12.1) 'To nourish the mind there is nothing better than to make the desires few. Here is a man whose desires are few - in some things he may not be able to keep his heart, but they will be few. Here is a man whose desires are many - in some things he may be able to keep his heart, but they will be few.' (Mencius 14.35) 'Desires should not be connived.' (Book of Rites 2.1)
Animalistics desires should be limited and restrained by developing our innate moral principles of benevolenve, righteousness, wisdom and in particular propriety (which Mencius suggested), and by using tools such as rites, music and fine arts (which Xunzi suggested). We should not indulge in them.
If a person consents for instance under false circumstances or false information they did consent but it isn't morally correct. A male or female who lies for instance is probably not in the moral right. Then you have issues relating to ignorance such as having an undiagnosed STD; you can't really consent to risky sex without knowledge of the risk itself just like any other contract.