The phrase "sexual union" refers to when two people come together to engage in sexual activity or have sexual intercourse. It means they are connecting physically in an intimate way. Full definition
The artist utilizes the simplicity of watercolor and colored pencil to celebrate the holistic pleasure of sexual union in nature. (arthaps.com)
It belongs to the created order of things that men and women should be joined in sexual union; neither gender is complete without the other. (religion-online.org)
Now that reproduction is possible without sexual union, some say that men and women no longer biologically need each other. (religion-online.org)