I will skip all the irrelevant "your mom" bullshit and the arguing.
I have forgotten more about deviant/kinky/unusual sex practices than most of you will ever know. This includes how to talk a reluctant partner into one or more of these. This is a simple fact that is not in dispute among those who know me well.
Now then, the answer can depend on wha the act is. Hate to say it but there it is. If we're talking backdoor pumping
m-->f or vice versa, there is no beating around the bush, no easy way to say it, but there is a right way and a wrong way to broach that particular subject.
If we're talking a little slap and tickle, that's easy, but full-on bondage is a different problem altogether. That requires a slow and gradual approach.
Then there is the whole horse thing. That usually requires an intimidating firearm and/or drugs and alcohol.
That all said, the general way to start the discussion is in a romantic, nonthreatening situation. Try a nice dinner out, complete with wine or drinks (just one or two to mellow her out, do NOT get her drunk!). During the coffee (or dessert) phase, try approaching the subject in a fun manner. "Don't throw her into a position of saying yes or no right away.
The idea here is to plant a seed in her mind, which will germinate and grow. The more she thinks about it and tries it out in her mind first, the less scary/intimidating/disgusting the subject at hand may be.
Women are like that. For whatever reason. Just plant the seed and change the subject, unless she wants to discuss it further. She will remember that topic and turn it over in her mind later on, maybe in the comfort of a bubble bath. Rest assured, she will not forget it. But if she comes up to you in two weeks and says "Remember what we talked about at dinner?" Well, I just can't go through with that. Then you have an answer. If, on the other hand, she wants more information, or wants to "talk" some more or flat out says "Let's go for it! Bend over baby!" You're golden.
Otherwise you may have to forget about it. If the unspeakable act you want is so important to you that you cannot go on without it, then you have a serious fetish and you need counselling...possibly marriage counselling. Wolf can better advise you on what to do in that case.
Good luck
Brian