I think the biggest problem is when one partner believes that the other is condemned for eternity in some way (not just a hell, but also other forms like reincarnation to a lower state or anything else deemed bad)…and either they spend the relationship trying to change the other person and probably ruining it, or worse, they accept that fate for someone they claim to love.
As an atheist, I don’t have that concern that my loved one is doomed to torment somehow, I just have the here and now to try and make their life with me as pleasant as I can. There is the issue of whether or not an atheist could live with someone whose rational is governed by beliefs that affect their judgement, either like mentioned above trying to convert them for their sake, or in other ways where religion steers them vs. having their own thoughts. But for what I think is a large majority, religious people mostly go through the motions if any just to fill some subconscious uncertainty and it’s not enough to threaten a relationship with a differing viewpoint.
The human brain is very good at compartmentalizing things to help us get through the day.
Yes. That’s why I said what I said in the parentheses, different beliefs have different afterlife ideas, some may not have a hell but might have other punishments. The point was a relationship where one or both thinks the other is going to suffer later but is okay with it can’t be very deep. If it’s a religion where there isn’t such a thing, then there isn’t a conflict (at least not in that sense).