So, what you are looking for is a sexual element? Does that mean you're interested in casual relationships, or would you also want a friendship or romantic attachment?
Or maybe you're not interested in getting sex from anyone but your wife? I'm not quite sure from your post.
At any rate, it sounds like apart from the lack of sex which is a problem for you, you have a nice relationship, loving and understanding, so good for you guys
Is your wife's relationship with her girlfriend in the open or is it more of a "secret" thing? Does she come over to your place, has she met you and your (step)children, etc?
I'm trying to understand the dynamics of the relationship, that's the only reason I'm asking, there is no right or wrong answer of course