I agree with Wolfi Landstreicher when he said in his article "Free Love" that,
The mechanics of erotic desire — homosexuality, heterosexuality, bisexuality, monogamy, non-monogamy, etc. — are not the substance of free love. It can manifest in all of these forms and more. Its substance is found in those who choose to expand themselves, to goad themselves to expand their passions, dreams desires and thoughts. Free love, like revolution, acts to recreate reality in its own image, the image of a great and dangerous utopia.
Thus, I would reject the very notion of an "ideal sex setup." In stead, I would argue that any situation in which each individual is a voluntary participant and is free to explore their desires unencumbered by the stale dictates of puritanical morality is consistent with an anarchistic ethic.
This, of course, raises the question of "consent" and what it actually looks. A lot has been said on this topic from a lot of different perspectives, and it can be murky territory trying to wade through it all in an effort to form your own conclusions. The perspective that I have come to over time is simply that, while every sexual encounter must be voluntary for all participants, consent is rendered in different ways that often don't take the form of explicit verbal statements.
The "Can I Kiss You?" trend that is currently in vogue in a large cross-section of the North American Left strikes me as overly paternalistic, stilted, and self-conscious. It tends to remove spontaneity as well as typecast people in the role of "victim." Assuming that no one has been physically coerced or emotionally manipulated into participating in a particular encounter, consent can emerge in far more organic ways. As long as you are acting from motives that respect both your own and the other person's (or people's) autonomy, then consent can emerge as a matter of course.