YMD,
Welcome to MB...what a profoundly brave and manly act you chose to do...seek help, find people who have lived what you are experiencing...and post.
My question to you is where is your respect for your wife?
If she says there is another guy...then say calmly and respectfully "I believe you. When are you moving out? I don't do infidelity. I do marriage."
Know what abuse is...this is abusive. She can say she intends it as a joke, and you are not laughing. It's not funny. It's abusive. Tell her what it feels like...state it, not demonstrate it..."When you say that, I feel stabbed, angry, and filled with a choking fear. I do not believe you are joking. I believe you are serious."
That's your reality...your truth.
If she continues with the joke...say, "That's abusive. I'll return in ten minutes to continue our discussion."
If she continues, remove yourself again, stating how long...make it two hours...and a time when you will return.
No one can "change" a subject, btw. You have yours and she has hers, when she switches. Continue with yours.
These are predetermined, progressive boundary enforcements. Use these steps progressively for the next time she does it. She may cease after the first one. That's great. If she does it again, at another time, go to the second one. Same for another day or week...go directly to the third one.
Your boundaries are yours. Taking abuse isn't in your standards, is it? Standards are what you do not allow yourself to do to others or yourself...boundaries are match your standards. Can't enforce a boundary if you're crossing your own standards.
Take an inventory of your humor, as well. Do you like and do sarcasm? That's abusive. What you may be experiencing is retaliation...punishment for her feeling abused...torn down...made fun of...find out.
Listening to her stuff...what she thinks, feels, perceives, believes and her perspective is all valid...it's hers...listen and acknowledge it. Share your own stuff.
Either way, a real A or not...or a thinking about one...comes from entitlement, fueled by resentment and lack of respect.
Inject respect...read all of Harley's articles on this website so you know how to thrive in marriage...look inside for where you may feel entitlement, resentment...you only control yourself.
Find the parts of what feels like stabbing and flip them over...do you feel lied to? Check if you're lying by omission to her...not knowing and sharing your stuff, for example. Do you feel like you're being replaced? Do you believe you CAN be replaced in life? Do you feel threatened? Are you threatening her in anyway...look for ways you may be attacking her and feel entitled.
When you flip over stuff...and look inward...you give yourself time to stop reacting and begin acting...from aware choice. This is the hallmark of an adult human...we choose everything...no one can make us do, feel, think, believe, perceive anything...we choose. And we know we choose.
As kids, we didn't know we chose.
You do now.
Take heart...keep posting, reading and learning. For yourself, your marriage, your wife and your child.
LA