There is no guarantee that things won't be awkward. What you can do however is signal that you don't expect the situation to be awkward. By doing this you will place social pressure on the people you're talking with to not be the ones to make things awkward.
The easiest way to make the introduction of your metamores feel like just another introduction is to make it just another introduction. Try to match your cadence so that saying "This is my wife and her boyfriend" is said the same way as how you would introduce your wife and her (platonic) friend. It's less about the specifics of what you say but how you say it. If you try to quickly change the subject, or pause and wait to see how people respond, people will pick up on that deviation from the normal flow of a conversation and assume that the deviation is relevant.
Depending on how out you want to be you can also prime people in advance by talking about your metamores when it's conversationally relevant. This will spread the potential awkwardness out, but it will make any introductions you make after the fact a non event since there isn't any revelation when introducing them. All that's happening then is just putting a name to a face.