You need to lay down the law here and stand up for yourself as a person who respects himself and expects respect from others. You say your marriage is going great but your wife shows no consideration for you as a person in anything you've mentioned! She tries to deny you basic human freedoms (no masturbation??? what is more your own than the right to your own body?) and feels fine with imposing on your privacy (reading your messages) but doesn't think *anything* should apply to her.
I think it was silly of you to put an open marriage on the table if you didn't really want it, but we all make mistakes and considering your wife's indiscriminate application of the idea (random men from the internet? does she intend to get to know them first or is this just about hook-ups?) and disinclination to take your feelings seriously ("railing" at you, really, how is that ok?), I can see why you'd be backpedaling now.
Ask her how she'd feel if you were the one who waited until she was away and then insisted on sex with a houseguest while being super shady about how much had already happened (hiding her communications with him). I assume from what you've said that she wouldn't stand for it. How can she possiblly lack the empathy to realize, then, how hurtful this must be for you? Tell her what a terrible idea this is -- she doesn't really know this young man, he thinks he's in love with her, and it's not fair to anyone to pursue this when you and she aren't yet seeing eye to eye about it.
There are a lot of things that need to change here. It is NOT OK for her to set these many double-standards about sex and then try to force you to be ok with something pretty extreme (sex with a virtual stranger -- why not a friend that you both know and trust??) right away, while hiding things from you! That should be self-evident! Is she really so blinded by her desires that she's lost track of right and wrong or did she just not get the distinction to begin with?
I invite you to invite her here to read my words. Or, of course, you could tell her yourself that the way she's treating you is ridiculous and that she's putting your marriage in serious peril. Stand. Up. For. Yourself.
The major players. Me, 30ish bi female. Gia, girlfriend of 4+ years. Clay, boyfriend/dom. Davis, ex/friend/"it's complicated." Eddie, roommate & fwb.
The supporting cast. Eric, Gia's husband. Bee, Gia and Eric's toddler. Dexter, Gia's lover. Helen, Eric's lover. Izzy and Nikki, Clay's partners. Liam, Eddie's husband.