When playing party games for couples, one has to know:
Since this is almost always the case, you ideally must provide this sort of space and privacy. Truth or Dare still is a perfectly fine party game for couples to play.
When throwing a big sleep over party, have all your guests sleep comfortably in the basement.
Prepare some blankets and sleeping bags.
For a couple however, put them in your room or guest room. Prepare a sofa in a separate room at the least. Show them that you care about their relationship and know that they need privacy. They will be happy to stay at the party longer.
Typically, one of the two gets tired first and the other partner follows. But when the boyfriend knows that his girlfriend is sleeping soundly in the next room, he may stay and play a little while more. Perhaps he will be less inhibited when his girlfriend is not around, who knows? Or, if the boyfriend leaves earlier, that means you have one more Truth or Dare girl at your party!
When playing Truth or Dare and when kissing is involved, you have to make sure no boundaries are overstepped for the couple. You don't want to destroy a relationship just because you played party games for couples. Even if everyone in the group treats it as a game - the couple may not!
However, if the couple was going steady for some time and is trusting with each other, you may include the couples even when kissing or stripping games are going on. You will have to discuss this beforehand,of course.
If everyone plays fair towards the couple and nobody wants messes with their relationship, the couple will bring in some new aspects. When a kissing dare occurs, and the couple dares each other, they may present quite some show, if they are out going!
In any case, inviting couples to you truth or dare party will not be a liability or a disadvantage. Just make sure you play it smart.
Don't let them retire in their cozy bubble by not inviting them to your parties anymore.
You can control how the party games for couples go! Expect of them to talk to the rest of the people. Encourage them to stay longer at the party. You can also playfully give them a little penalty: say "if you go to sleep early, you will have to help tidying up the next morning...!"
Probably, they will agree!