I am definitely not a lawyer, but I think conviction for indecent exposure/lascivious conduct often requires someone to actually witness the perpetrator(s). Since no one actually caught them in flagrante delicto, that condition wouldn't apply. Other things that aren't issues, off the top of my head:

• They didn't access the building unlawfully
• All parties in the video consented to both the sex and the recording
• D.C. no longer has any sodomy laws
• Video wasn't filmed/distributed for commercial purposes

It's just hard to identify a harmed party when you think about it. It'd be nice to hear an analysis from someone who understands the D.C. criminal statute and its interpretation, though--I wouldn't be surprised if their conduct is illegal in other jurisdictions.

Not authoritative, but I did find this page from a D.C. law firm which states:

Indecent exposure requires exposure at a time and place in which a reasonable person would know that his/her acts will be open to the observation of others.  Parnigoni v. District of Columbia, 933 A.2d 823 (D.C. 2007).  The accused does not need to have a specific intent to expose himself to any particular person.  It is sufficient if the defendant generally intended to expose himself so as to draw attention to his exposed condition.  Id.  

The required criminal intent for indecent exposure is usually established by evidence indicating that the defendant attempted to draw attention to his exposed condition or by display in a place so public that it must be presumed it was intended to be seen by others.  Hearn v. District of Columbia, 178 A.2d 434 (Cr. App. 1962).   In other words, surreptitious masturbation in a public place — without more — might not satisfy the requirements of the statute.  

If I remember the facts of the case correctly, they were in a hearing room not accessible to the general public. Presumably this was taken at a time where there was no hearing scheduled anytime soon and no reason for anyone to enter that room, so it would be difficult to establish the intent to expose themselves to someone physically present.

(Side note: when I was looking for more info I saw that most of the posts on reddit were from right-wing subreddits with titles that made sure you knew it was SODOMY and ANAL SEX. The horror!)