Having relations without first immersing in the mikveh is punished by kareit, which is a grave punishment (it also applies to violating Yom Kippur, Shabbat, idolatry, adultery, incest, etc.) The Rambam writes as such in MT Isurei Biah 4:3
If, however, a niddah, a zavah or a woman who gave birth did not
immerse in a mikveh, a person is liable for kareit for having
relations with one of them even several years afterward.
The prohibition and the punishment is incumbent on both husband and wife equally as the Rambam writes in MT Isurei Biah 1:1 based on Vayikra 20:18
When a person voluntarily engages in sexual relations with one of the
arayot mentioned in the Torah, he is liable for kareit [...] The
plural is used, referring to the man and the woman.
In a case where the couple is not planning to have relations, going to the mikveh does allow them to touch each other, pass on objects, sleep in the same bed - all of the prohibitions when a woman is niddah (knows as harchakot). And it would enable relations should things change.
PS. See here in a case where the husband is away.
Of course, consult your rabbi
before implementing anything you learn here.