While infidelity is technically still a crime per UCMJ for persons who have signed a UCMJ contract, my understanding is that infidelity is very rarely prosecuted
AFAIK the president is not subject to the UCMJ despite being the command and chief otherwise there are many actions by many presidents I am certain would have them in front of a military tribunal. A sitting president can not be court-martialed. As a side note I knew a few people that were caught cheating when I was in the military and their lives were turned upside down by the Chief Master Sergeant.
Clinton was impeached for perjury about infidelity, but not for perjury about not inhaling.
One of those is really hard to prove unless tested immediately afterwords. On the other hand ejaculate on a dress allows for genetic testing. All recent presidents DNA are on file.