Can swallowing semen cause mono?

No: Mononucleosis (http://www.mayoclinic.org/diseases-conditions/mononucleosis/basics/definition/con-20021164) is caused by Epstein-Barr virus (EBV), not swallowing semen. So swallow as much as you want, assuming you're in a mutually monogamous relationship & not at risk for sexually transmitted diseases. If not in MMR, then use protection each & every time to avoid exposure to body fluids incl semen

Unlikely: Haven't seen an actual research study in which semen samples from EBV-carrying guys were tested for infectious EBV viruses. Generally, people catch EBV (the mono virus) via saliva. Since kissing generally precedes swallowing semen, in a typical relationship, the virus is likely already transmitted via saliva, long before any semen is swallowed. Thus, no worries about catching mono via semen.