It depends on the age of the couple, how long they've been dating, how attracted one is to the other, and whether or not the coupon was mentioned in advance.
Two college students aren't likely to be offended because one has a 50% off coupon, nor is a couple who have been dating for a few months going to feel ashamed to take advantage of a bargain.
Generally speaking it's a "no-no" on a (first date). However if someone were to say (in advance); "I got a 50% off promotional coupon for that new restaurant that opened up. Are you interested in checking it out?" Odds are the other person would not be offended. In fact if someone thinks you are "HOT" they could care less about the coupon or the restaurant. They just want to be with (you) period!
It's when people try to sneak the coupon in during the date, tell you which entrées to order, or state: "We must get our order in before 6PM"...etc (after) you said yes to the date; it comes across a little slimy. You're better off cooking a meal at home!