A lemon is rated R. What makes a lemon is usually a hinting at an adult situation. In other words, if you wanted two characters to make love in your story, then you wouldn't write down the actual act, you would hint at it. Even actually write down the act, but not in an explicitly adult manner. For instance:
I had never felt so free before. Making love to him/her was the best experience I had ever had in my life. I felt as if I could take on the world.
Also, you may have the characters undress in the story, but as far as you can graphically depict them would have to be kissing, and carressing. A good author's tip, use metaphors. Or look above at that sentence I wrote, just hint at them making love, don't show the actual act. Just show before and after.