If you ask most people who have been married close to 50 years, they will still tell you that the secret to a happy marriage means making an effort.
Great sex doesn't just happen. You have to make it happen. Both of you. If either one of you just lie there like a dead fish, it's over.
Thankfully I know plenty of long-married, happily-fucking couples. My favourite were the parents of a girlfriend. They lived in a small town in Central Ontario in a big house the husband built himself. When he built it he specifically ensured that all the walls were insulated -- not just exterior. Reason: So he and his wife could have as much loud sex as they wanted without fear of waking up the kids.
When the girlfriend and I visited one weekend, the two of them took off in his truck for a few hours. When I asked my girlfriend why, she said it's because, when the weather is nice, they like to drive out to a secluded field, lay down a blanket, and make out like teenagers.
It's this kind of inventiveness that keeps a marriage interesting, I realized.
As to what kind of nastiness they get into -- bondage, role-playing, anal -- I have no idea. Maybe none of that. But I notice that a lot of these couples I know still wink at each other at the dinner table and such, and give each other spontaneouus hugs and kisses. They're obviously up to something.