Dick's all "I'm bad for you because everyone I know dies!" and Tarantula, while definitely skeezy in her opportunism, is I think genuinely trying to prove that she thinks he's the greatest thing since sliced bread in the most immediate physical means she can. There's no sense from what's presented that she's aroused by killing others or unaware of his attitude towards taking life (though as later seems the case, she underestimates its intensity) -- check out the grim expression on her face as she shoots Blockbuster, she's just doing her "job" as she sees it needs doing, killing an irredeemable monster.
The dialogue on the roof basically starts from the point where Dick is saying he failed her which can quite easily be construed from Tarantula's POV as the failure being he's claiming ownership of his inability to "man up" and kill Blockbuster himself and thus forcing her to do so. Dick isn't saying he doesn't want sex, he's saying having sex with him is a bad idea, that he's toxic.
Which only reinforces the idea that from her POV she's operating morally, and simply denying the validity of his claims to failure by giving him the reward of sex -- and sex will not ever not be a reward -- thus physically demonstrating to him his own manliness and her not considering him a failure. There's no sense that she's under the impression she's forcing sex on him -- at worst she's simply persuading him he wants it. Rather you're in drunken/traumatised sex territory of dubious consent here which is probably what Grayson meant to say/should have said.
Even afterwards, Dick doesn't seem at all bothered about the sex but rather the killing, desperately trying to persuade himself that he ultimately hadn't wanted Blockbuster dead.
Trying to appropiately cast a genderflipped situation (something I found Cry for Justice's comparisons didn't really succeed at) leads to the interesting observation that female heroes in the DC seem statistically more willing/likely to kill than their male counterparts.