Iron Man's Tony Stark is known for being an iron man. If you trace the idea that when you sleep with someone, you're sleeping with everyone your partner ever slept with, Stark's penis reaches Galactus.
Wizard Universe Magazine tried playing "Sex Degrees of Tony Stark". It's pretty big, and it's only gotten even more complicated in the years since.
With a name like that, you'd think most guys would want to keep their distance.
The very first story of her short-lived ongoing series revealed the KGB infected her with a dormant mind controlling STD way back when. Since she's been with some of the biggest man-whores in Marvel, this turned out to be a very bad thing.
Hawkeye also only gradually grew into his reputation as a womanizer, as during the Silver Age he really only was just involved with the Black Widow (it did not end well) and then the Scarlet Witch (unrequited).
It's not a Fantastic Four comic unless Johnny Storm is either on his way to, in the middle of, or recovering from some supermodel date.
Thanos of Titan has had hundreds of mistresses all over the Universe. he killed them all later.
Matt Murdock, referred to by Wolverine as the "biggest him-bo" in the Marvel Universe. Which was pretty hilarious, given how much Wolverine gets around.
In one Spider-Man story, Loki recruits Spider-Man's help in dealing with a woman who is a victim of Demonic Possession. Later, Loki discovers that the woman is his daughter. When Spidey is surprised that Loki has children, Loki says that he has hundreds. ("I'm a Norse God," he says. "It... happens." Clearly, he's very much like his mythological counterpart in this regard.)
Spidey himself is no slouch in this department either. While not near the number of lovers Tony Stark, Daredevil, or Wolverine has amassed, Peter for a supposedly unattractive nerd has had quite a few lovers: Gwen Stacy, Mary Jane, Felicia Hardy, Debra Whitman, Michele Gonzales and Carlie Cooper are just the tip of the iceberg. What makes Peter perhaps exceptional in this regard is that he is actually serious about his relationships with his sexual partners (only confirmed to have had sex with a few of the above) and was even married for a long time. and most people would prefer he still be married, making Peter one of a few characters on the list who people prefer to not be this trope.
She-Hulk: It's become a running joke that She-Hulk is very... liberated. But she did not sleep with Juggernaut! Well she did, but it wasn't her, OK?
Her Skrull friend Jazinda is similarly uninhibited. When She-Hulk asked her how many people she'd had sex with she replied five. Then asked to confirm that she had meant at the same time, right?
Jen's also gotten quite friendly with Starfox, whose real name is Eros for a reason.
Jen was once ordered in court to list off every sexual partner that she (as She-Hulk and not Jennifer) ever had. Without pause, it took her 20 minutes to list off everyone.
The Incredible Hulk: Although far less than his cousin, the Hulk does get more affairs than expected for a giant, hulking ball of pain. From Betty, Meriam (a siren), Jarella, Bereet, Kate Waynesboro, Marlo, Caiera, and the college girlfriend that he saved from being unlawfully contained in an Amnesty International condemned US prison; to being desired by Thundra, the other college fling Monica Rappaccini and Umar (TWICE)... Not to mention Joe Fixit who probably had ten times as much sex as all the other incarnations put together. Lampshaded by Red She-Hulk, in those Exact Words. He even has a couple of kids as a result of this (at least Jen uses birth control, it seems).
Mystique has a long list of ex-husbands and an even longer list of people she has slept with. Her official stats page lists two sons and two ex-husbands, neither husband being the father of either child. Her list of conquests is so long, it'd be easier to list the people in Marvel she hasn't slept with. The fact that she's a Serial Rapist doesn't help matters.
In the story fleshing out the backstory of her son Nightcrawler, she was married to the head of the house, sleeping with the driver while pretending to be the maid, and then started sleeping with her husband's best friend. It was also implied she'd slept with her fertility doctor when she had trouble having children with her husband.
Azazel too, given that he has had dozens of children by multiple women, including Mystique. Ironically enough, their son is Nightcrawler.
She-Hulk's male counterpart in the Marvel Universe is Hercules. They're very good friends. His other known very good friends include the Black Widow, Snowbird, Namora, Alflyse, Psylocke, Nova, queen of the Dark Elves, and quite possibly Northstar. He also owns hundreds of properties around the world, each of them home to a former lover. Since he is an immortal that has traveled the world since Ancient Greece, various historical figures/gods/regular women have probably been left off his full list.
Speaking of Wolverine up above, he's been around since the 19th century, in more than one sense. An entire plotline in the comics centered around a villain gathering up as many of Logan's unknown illegitimate children as he could find and sending them to attack him so he would kill them all as part of a revenge plot to screw with his head. Wolvie may get more tail than Stark.
. It's pretty big, and it's only gotten even more complicated in the years since.