What do you mean by feeling exhausted?
It doesn't sound like seeing happy couples/not having sex is making you exhausted. It sounds more like it makes you sad to see, maybe even jealous, depressed, self-conscious, or frustrated, and those feelings wear you out. Does that sound right?
It would align with how eating and exercising hasn't beem helpful for you. You need to give your brain energy by making it focus on other things. Try doing mindfulness, start by staring out your window and looking at people, then go outside for short periods of time. Keep extending that time until you can get to a couple hours. Restructure your thoughts, instead of thinking about your own loneliness when you see a happy couple, focus on the trees instead. Think about how the trees are green and have branches, count the leaves, whatever. The important part is creating a distraction in being present, so that you don't let your mind wander off to the feelings of loneliness.