I have to say, that was always my impression of Nightwing/Huntress too. I really never thought he was sleeping with her at all because she reminded him of Bruce (hell, if there's somebody he's slept with who's Bruce-like I think Babs fits that bill far more than Huntress, who doesn't remind me of Bruce at all). Iirc, didn't she basically just offer it? I seem to remember her basically saying "Don't you ever get..." And Nightwing said "Lonely?" Seemed more like two single people wanted to have sex that night and there wasn't much reason they shouldn't. (I thought it got stupid with Barbara calling and identifying that he was there--this and that Nightwing annual seem to be where the manwhore stuff comes from and he just isn't.)
Anyway, also with Tarantula I've never gotten the Mary Sue thing because she always seemed so completely wrong, but I guess what people mean is that Dick for some reason sticks with her in spite of that. Like she's given way too much power over him for who she actually is and all that.
Was it Babs who actually put it in terms of Helena being like Bruce? Because that could just be some massive projecting. She's got a lot of opinions on Bruce/Dick (none of them sexual!) that have a lot to do with her own frustrations with Bruce, I think. And it would be IC for her to feel that when she's angry at Helena for the incident too.