Bistro lights for wedding structurally sound?
This will work fine, with these caveats:


The posts will certainly be strong enough. The question is whether the soil surrounding their concrete bases will give, allowing the posts to tilt. Worst-case scenario you add guy wires down to the ground as support.
Your primary (perimeter) wires should be 1/8" steel wire rope, at least, but light enough to not add to the challenge. You'll need as much as several hundred lbs. of tension on them (with the cross wires in play) to reduce sag far enough. Synthetic winch rope could also work well, but won't be cheap.
Your cross wires will also need to be able to handle 50 lbs. tension or more. Picture wire might do, or 1/16" steel wire rope. 


If you can accomplish all that I think you'll have a fantastic wedding scene. Good luck, and GOOD LUCK. ;)