More findings: Semen seems to be a good decongestant. I breathe much easier after applying it. I was worried that overuse might cause rebound congestion, but according to the Mayo Clinic webpages, nasal steroid sprays don't cause rebound congestion. If semen works like a nasal steroid spray, it's ok for long term use.
Anyone whose already congested should first clear the nasal passages with saline solution in a Neti pot or nasal irrigation syringe.