You might explain it to your wife like this:
Judaism makes the most important things in life mandatory rather than optional. It is not optional for a Jew to pray. It is not optional for him to have children. It is not optional for him to give charity, to love people, to study, to eat, to drink, to rest, to fear G-d, to remember the past, and to look every now and then at the sky above.
For the things that would seem expendable for being overly self-indulgent--rejoicing, resting, prayer, sex--Judaism sets fixed times. We must rejoice on holidays. We must rest on Shabbos. We are commanded to meet G-d in prayer three times a day. We stop everything to do these, regardless of our schedule, our feelings, and the literally endless exigences of competing obligations. Because we stop, we do not waste our lives. The most important things in life are given their place. We use our short time on Earth wisely.
Judaism seems to me adamant that sex with one's spouse--with its exquisite pleasure as well as its procreative potential, mystical significance and inherent holiness--is one of the essential elements of a good life. Therefore sex is on the agenda, literally. Like Shabbos, Rosh Chodesh, holidays, davening and most every other time-bound obligation in Judaism, its scheduling follows natural rhythms. In this case, the arbiter is not planetary movements, but a much smaller, though equally ecological system: the cycle of the wife's body. As with the other mitzvos, we bind the natural shifts with their meaning to us through rituals. In this case, the ritual is the wife's immersion in mikvah. After that, couples make literally every effort to ensure that the night is a special one: This most important thing in life happens, without fail.
I wish your wife could see it this way. And perhaps your wife should visit a new mikvah every month until she finds one where the holiness is palpable to her. Maybe it is in the snow or in the ocean. Maybe the secret for her will be to escape from rules, from niggling supervision and from arbitrary social norms. Maybe she needs to see it more as connection with G-d. I am confident that once she does, she will not depart from this mitzvah.