Pesachim 72b discusses a woman who desires intimacy from her husband, and it is approaching the expected time of her period.
והאמר רבא חייב אדם לשמח אשתו בדבר מצוה סמוך לווסתה
The Gemara asks further: Even so, didn’t Rava say that a man is obligated to please his wife through a mitzva? That is to say, he must engage in sexual intercourse with her when she so desires, even if it is not the time of her conjugal rights. The Gemara answers that we are dealing with a case where it was near her expected date of menstruation, when sexual relations are prohibited due to a concern that the woman may already be menstruating or that she may begin to menstruate during the sexual act.
Rashi and Tosafos there explain that in Shevuos 18b it is derived that one must separate from their wife as she approaches the expected time of her period.
I would infer if one must separate from their wife just before they become a Niddah, even if she desires intimacy, all the more so when she is actually a Niddah.
It's obvious to me that the obligation to one's wife doesn't include times where she is prohibited. I'm not sure if I need a source for that. It's not like this prohibition (which incurs kares) overrides that prohibition (really, obligation). It's that there is no obligation, just like when she's not interested there's no obligation.