Suppose you became deeply religious as a young adult

This assumes that different kinds of religiosity tend to converge on similar ethics about marital commitments and fidelity. You could become "deeply religious" in a way which allows for divorce or outside relationships.

This also assumes that your religion's doctrine on these matters remains stable over many generations. If your religious community accepts 22nd+ Century medicine and permits its members to seek treatment for engineered negligible senescence and superlongevity, then you could live long enough to see your religion undergo a Reformation-like event which allows for a more flexible view of marriage and sexual relationships.

I think I've mentioned this before, but I find Ridley Scott's portrayal of Future Christians in the film Prometheus interesting. The space ship's archaeologist character, Elizabeth Shaw (played by Swedish actress Noomi Rapace), wears a cross and professes christian beliefs at a time when christianity has apparently gone into decline and christians have become relatively uncommon. Yet as a single christian woman she has a sexual relationship with a man on the ship, which suggests that christian sexual morality during that religion's long twilight will tend to converge with secular moral views.