0% chance I would let my wife - before or after marriage, go away travelling for 6 months.
(Before people scream sexism, there is also 0% chance she would let me do it either)
Why? Because we are in a commited relationship, and our lives and futures are together. 6 months is a long time, and travelling is an amazing thing that can bring huge moments in your life. In my opinion, if your in a serrious relationship, that you want to last, those moments should be shared together! Not apart. If you want your relationships to last, you need to work on fostering them, and developing them, and a part of that is both intertwining your lives, and creating an abundance of shared experiences. Travelling alone does neither.
I would question, why its nessasary for her to travel without you?
All I can see is either:
1, she can go now, and you cant go now due to circumstances. In which case, why can't she wait for you? Maybe do something small now, and save the 6-month long life-changing trip until you can go together.
2, she wants to go, and you don't.. well thats a pretty big signal that your priorities are probably not in the same place..
Neither of those are good signs.
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I haven't even mentioned the cheating. Obviously there is a increased risk of her cheating. But that for me is just a small part of the much bigger problems I have with a trip like this.