In that perspective, that a husband and wife find out together what they like to do in bed, with respect for each other, is a lesser problem. A passion is a passion, even between spouses, and ideally, alternative forms of sex which are triggered basically by passions, would not exist where passion does not. But, as it's been said, this is a war, and in a war, many times, we have to choose the less bad of two options.
Is a passion a passion?
Love, even sexual love (Eros), is not necessarily sinful in itself. Isn't it the perversion of the emotion, to desire for the sake of desire, devoid of love, that is sinful.
Another example would be anger. Anger in itself isn't necessarily separation from God (sin), it's the hate and rage from giving into the anger that produces the separation.
Spousal love is Eros. And Eros is not only the sexual desire we can feel for our spouse, or, in fact, for just about anyone that fits our "standars" as a sexual partner. Eros, in itself, is not at all sinful, and nor is, by the way, sexual desire. Sexual desire is just like hunger, something that "happens" to us and if we treat it like a pet, "teaching" how to behave it's not a problem.
The passion problem, in patristic terms, is that what is natural may become a compulsion, or may be directed towards things it was not meant to be directed at. So, sexual drive can be directed toward specific parts of the body (legs, eyes, smiles, hair, feet, etc) or of the personality of the person (intelligence, humour, pride, social status, vices, virtues), while they should be directed at the whole person. They can be directed at people of the same sex, of both sexes, of different ages, at animals, plants, objects or at a different new person each time. It can be addicted to certain things like power-games, images, role-plays, pain, humiliation. It can be directed at nothing in particular just needing to be "felt" by whatever means. And it can be any combination of the formers.
These are all "catamerisms"of sex, where something that is meant to be integrated in the committed love of the whole person is, unfortunately, attached to a part of the feeling itself, or the imagination or of the person. It's shattered desire, weakend. It's like a person who has lost the sense of taste and thus require even stronger and more exotic spices to just be able to feel something.
The "danger" in any form of alternative sex is that it *can* escalate and numb the very sense of sexual pleasure it supposes to be stimulating. Sex is pretty much like alcohol. It's pleasurable, in small dosis it's even healthy, but it can and often leads people to very dark paths. Plus, it will die with this body. So it's not a trustable pillar for a relationship. Only love is.
So, for example, if two spouses start relying on sexual satisfaction for their relationship, it may happen that one of them will have desires the other does not want to satisfy. It may destroy the relationship. Or worse, the corruption of one may drag the other and that spousal love that was made to make them both rise to heaven, will make them descend even faster to hell.