Harley Quinn creator Justin Halpern revealed the team had to cut a Batman scene from the show - but deleting this particular scene only highlights the strange ways in which the Dark Knight is handled. In an interview with Variety, the Harley Quinn creators discussed the various highlights of their time working on the adult DC cartoon thus far. They covered the difficulties of adapting comics into different formats, most notably referring to the careful balance that has to be struck when using such a carefully crafted and protected franchise for an intentionally outlandish and brazen comedy that lovingly mocks its own source material.
Halpern stated “A perfect example of that is in this third season of ‘Harley’ [when] we had a moment where Batman was going down on Catwoman. And DC was like, ‘You can’t do that. You absolutely cannot do that.’ They’re like, ‘Heroes don’t do that.’ So, we said, ‘Are you saying heroes are just selfish lovers?’ They were like, ‘No, it’s that we sell consumer toys for heroes. It’s hard to sell a toy if Batman is also going down on someone.’” While it was made clear that the creators were encouraged to push the boundaries of what could be included in the notably violent and sharp comedy, it stands out as especially weird that this is where the proverbial line in the sand was drawn for the series. This is doubly true given the series already contains a full storyline wherein Poison Ivy and Harley Quinn have sex multiple times - though, as previously stated, apparently this concept only applies to heroes.
While sexual acts obviously aren't a PG affair, neither is Harley Quinn - in fact, it's a cartoon very clearly aimed at older audiences, with enough gore and F-bombs to ensure any unknowing parents quickly realize they should pick another show for their kids. Other Batman cartoons have even had significantly more raunchy deeds happen with Bruce himself - as somewhat infamously, the Dark Knight and Barbara Gordon have a sex scene in the cartoon adaptation of The Killing Joke, despite the fact that conventionally the relationship between the two is an almost paternal one. The Killing Joke also contains scenes of a fully naked Commissioner Gordon, weeping and traumatized, as he is shown essentially a slideshow containing his daughter's brutalized (and also naked) body, after Barbara Gordon has been paralyzed by the Joker himself. It's clear that sex and violence of a similarly graphic nature aren't off the cards for Batman, making it all the more bizarre that a scene which by all means appears to detail two consensual adults - who don't even have the questionable dynamic of Barbara and Bruce - is somehow deemed inappropriate.
This is especially confusing given the more violent example, as the idea that a scene of Joker seriously harming and then stripping Barbara is more acceptable than Batman having some consensual oral sex with Catwoman is somewhat unsettling - as one scene is significantly more likely to traumatize its audience, or bring up their potential existing traumas. Violence is to be more expected in the likes of superhero franchises, of course, but ideally that comes hand in hand with showing a variety of healthy dynamics and relationships where possible, instead of suggesting that sexual activity is somehow more taboo than murder and bloodshed - at least, when it comes to big-name heroes. Harley Quinn has actually addressed this idea previously, as in season 1 episode 4, Batman has a heart to heart with Damian about waiting "until he is ready" to find a nemesis to beat up - only to immediately dart for the door when his son tries to have "The Talk" with him.
Figureheads of franchises such as Batman and Superman are naturally subject to some very specific rules regarding what can and cannot be done with them - and this is doubly true in the world of film and TV, as superhero comics are expected to and do contain more adult moments and storylines. However, Batman has long been the poster child for grittier and darker imaginings of superheroes, which means it's somewhat silly to imagine concern over whether him performing oral sex on Catwoman would negatively affect his merchandising rights - especially given Harley Quinn is a comedy cartoon, and thus likely not the most canon edition of Batman and whatever his sexual habits are, if indeed a canon version of this exists.