Green Arrow of The DCU has a mostly undeserved reputation for having trouble keeping his arrow in his quiver. While it is true that Oliver Queen was every bit the player Bruce Wayne pretends to be before becoming a superhero, he was characterized as being downright devout and at times downright possessive regarding then-girlfriend Black Canary throughout the Silver and Bronze Ages of comics. His characterization as a womanizer didn't begin until shortly after his death in the mid-1990s and he never cheated on Black Canary until Judd Winick started writing the character.
Roy Harper (aka Speedy I, Arsenal, Red Arrow) was Green Arrow's sidekick/Ollie Queen's adopted son back in the day and apparently whatever Ollie had rubbed off.
Green Lantern Hal Jordan as well. He's been shown having a one-night stand and being very flirtatious in general in the current Green Lantern book. Maybe there's something to that green M&M rumor.
Or the rumor about that one night with Huntress, Lady Blackhawk and a bottle of winenote In the Birds of Prey issue where this is brought up Huntress and Lady Blackhawk leaving a drunk Hal Jordan behind in a strip club. It's likely that Hal just got with strippers cosplaying as them.
There's also the time Hal Jordan slept with a 14-year-old alien. Most fans don't like to talk about that.
There is also the implication that he slept with John Constantine.
Both Mark Waid's Flash and Green Lantern: the Brave and the Bold miniseries (set at various points in Hal and Barry's careers) and Grant Morrison's The Green Lantern: Season Two have a Running Gag of a different pre-existing ex-girlfriend of Hal's every issue.
Ayla Ranzz (Lightning Lass/Light Lass/Spark/Gossamer) in Legion of Super-Heroes. This is a mix of Informed Ability and fanon; as this (somewhat outdated) list shows, she hasn't had more relationships than the other major Legionnaires.
The list was pre-Threeboot, which has taken the concept and run with it.
Huntress, played up especially in Birds of Prey, with regards to the dialog between Nightwing and Arsenal above. In an annoyed Black Canary's words:
Black Canary: All right. I'm sorry I implied she was a brazen hussy on a twenty-four hour man patrol.
Batman himself is something of a subversion: While he has definitely had a lot of sex partners (Catwoman, Talia, Vicki Vale, Julie Madsen, Jezebel Jet, Silver St. Cloud... and possibly Zatanna) for a guy who is supposed to be a bitter loner, it's nowhere near the Starkian number that people think billionaire playboy Bruce Wayne has had. In one comic it's mentioned that trying to make people think this is one of the more difficult parts of his masquerade.
His protege Nightwing also has a reputation for this. In reality, while plenty of women are vocally attracted to him, his actual dating history is a lot shorter. In fact, pre-Flashpoint, dialogue had Dick express that he was only sexually attracted to people he was emotionally intimate with first, indicating he's actually demisexual (IE, the opposite of this), and was used to contrast Roy Harper's casual sex addiction. This aspect has been forgotten as his Mr. Fanservice nature has gotten more mileage, leading to him hooking up with women a lot more casually.
Wally West, the third Flash, has a reputation as a skirt-chaser, one he earned during his early days as the Flash (where his behaviour was such that Power Girl named her tomcat after him as a taunt). This phased out when he fell hopelessly in love with Linda Park, developing a Single-Target Sexuality so fierce that even when he loses his memory of her, he has no desire for romance with anyone. In fact, he actually regards his playboy days with a lot of shame due to regretting how he treated women back then. In his case, it appears to have been the result of his father Rudy setting a terrible example of how to treat women (as he serially cheated on Wally's mother and often hooked up with sex workers), alongside Fridge Horror of Wally processing the violation he felt after Raven used Mind Rape on him to make him love her.
(somewhat outdated) list shows, she hasn't had more relationships than the other major Legionnaires.