Originally Posted by boutrosboutros
If you've been together for 7 years surely you know better than any generic advice anybody here can give.
That's a pretty good answer, TBH. But if you still want generic advice from a stranger: honesty and straight talk.
A lot of couples get by without talking much about sex/relationship stuff, because society gives us some expectations on how relationships are going to work. You don't have to tell your partner "I promise not to sleep with your best friend" because that's the default model. It's sort of like walking into a shop and buying a PC off the shelf - even if you don't know anything about computers, it's pretty easy to find something that will handle standard tasks without catching fire and exploding.
When you get into nonmonogamy, you're leaving the default model behind. You have
to be able to discuss things that are important to you, even if it feels awkward and uncomfortable, because otherwise your partner doesn't know what you're expecting from them. (As the saying goes: "telepathy failures are never the fault of the receiver.") So this is a good place to start practicing those communication/negotiation skills.
"Hey, have you ever considered a threesome with another guy? Is it something you might be willing to do?"
It's not a risk-free approach, but as far as I've seen it beats the alternatives. It's possible that her answer will be "no, I don't ever want to do that" and if it is, that answer has to be respected.