That's why you make sure to address those things. Make sure to emphasize that you care deeply about her and that's why you want to share this experience with her, IF it's something she's open to. Approach it like a fetish: this is something you desire, but only if she consents. You two can work out the details or an agreement or whatever if she's open to it.

Alternatively, you could ask about opening the relationship in general if you want to ease into it softly, but that could be considered dishonest if the only reason you want to open the relationship is to see her have sex with someone else.