I would never judge anyones relationship or style because "one size does not fit all.... But I would think subjects such as monogamy and fidelity would be discussed way before any "committment" to each other is made so that its clear where both partners stand when in the relationship. YES, things can also change over time, and here is where communication becomes key. For me personally, it wouldnt be comfortable being in a relationship if I wasnt on the same page as my partner... I dont like secrets, and I'm a bad liar, so relationship transparency is crucial for ME.....
Hiring, just like affairs is "addictive" behavior, so I dont see where one is preferred over the other, and in either scenario it would just be more comfortable doing "openly". My belief is that if you and your partner are not on the same page, or able to find some common ground, its just the wrong relationship for you to be in.....
Secrecy can be very destructive to a relationship. It changes your behavior, your openess and your intimacy, however if these are things you can deal with and handle, then you need to do what works best for you...
Deception is a slippery slope, so make sure your shoes have traction......