The old "If you don't do what I want I will say you did it anyway." ploy. Better to refuse as there will be evidence of the deed if you go through with it.
Twins used "Double Team Sexy Time" combo on Terakoshi-sensei ... IT'S SUPER-EFFECTIVE!
But Seriously, though, and IRL, Sensei has become (figuratively) a toy for a pair of brats, with no guarantee that, if either or both get bored with him in the future, they won't lay the 'false accusation' smackdown, with the typically expected consequences (BRR). At that point, his only way out, aside from 'six feet of earth' is if their mother *koff-koff* takes an interest in him.
Then again, back in the H-Universe, that latter option might not be so bad ... in fact, it's not altogether impossible that Okaa-san "Knows" (say-no-more, say-no-more).
"I'm sure they'll lay off me eventually"? Probably, once they get what they want from you...
In the end, they resorted to sort of blackmailing him into sex, but hey, I think it all worked out rather nicely.
He gets paid, their grades improve, and he also gets sex with two cute twins? He's landed quite the gig, I'd say. Win-win all around.