I agree with Kevin, this is her concern, not yours.
And........have you ever perhaps thought that you
are "the one"? She may not meet any other man she wants to commit to as she has to you. That is ok, not everyone is meant to have multiple grand loves. She is having fun, leave her to it, I think you may be a little jealous but instead of making the onus on her to make you happy by asking her or more threesomes (seriously???) take responsibility for your own happiness, get out there and meet some more people and have some of your own fun.