John has twenty years on Pamela, and she's not his type. Too entangled with the supernatural and too brunette. Pamela won't tell John that his Mary was just as caught up in that net as he is, and she doesn't try to kiss him, never mind fuck him. She does spend a few happy hours fantasizing about it, though.
Fucking Ellen would have been a bad idea all round. Mostly because if Ellen and Pamela were fucking, Ellen wouldn't have been serving drinks when a particular hunter walked in and therefore that hunter wouldn't have gotten the information he needed to save five lives in Alabama, but there was also Ellen's husband and daughter and that wouldn't have ended well for anybody.
If not for the angel, Pamela would have fucked Dean. Foregone conclusion. Doesn't take a psychic to read that sort of aura. If not for the demon, Pamela would probably have fucked Sam. It'd depend on whether the prospect of naked fun times was tempting enough to get him willing to push past his inhibitions at the thought of naked fun times with somebody he's not attached to. And once she'd fucked them both, it wouldn't be at all difficult to maneuver things so she was fucking them both at once. Sam once saw Dean fucking a set of twins, and once said Dean's name while fucking his best friend and later had a threesome with that friend and Sam's girlfriend, so it's not like either of them had any insurmountable objections to the idea of an incestuous threesome, no matter what Dean said about it.
(Lucky for Sam that Dean's a psychic null zone. He'd have no chance of keeping secrets from him, otherwise. Sam thinks loud.)
When Pamela fucked Jessie, she was eighteen and stupid. Sue her. The tattoo artist was twenty and close-minded. If Jessie's parents hadn't found out and flipped their lids over that girl corrupting their daughter, Pamela might have sued him.