Date night, once a week if you can afford a sitter. You are better parents when you are a better couple. There are great sitters out there. You can do something free if you can't afford a fancy night on the town. Free concerts, free walks, almost free movies, coffee shops, the little things. You need time apart from your children to stay strong as a couple. I have to disagree about sitters being hard to come by, as I am one and know many more. Ask friends for their sitter's number. Screen your sitter, interview, check references, whatever it takes to be sure your children are safe, but also when you do find a good sitter, treat her well and she'll be there for you.
Don't wait until going out is for fighting. Make time together away from the children a priority. A stronger, better couple makes for a stronger, better family and is best for your children. If you want to make it happen, you can. Trade off with a friend, if you don't have family around and can't afford a sitter. Just make the time to go out alone a weekly habit, an obligation to your children's future welfare.