Earlier quoted context omitted.
Caffeine has no smell so what you are detecting is some other component or effect on the semen taste. Many things can affect the taste of semen.
He didn't specify difference between de-caf coffee vs regular - only coffee vs non-coffee - so might be something other than caffeine that effects smell?
Coffee contains a large number of aromatic components that could be transferred to the semen. Just the way methyl mercaptan from asparagus can be transferred to your urine.