How do people end up in "dead bedrooms"?

I’m genuinely curious—not trying to offend. If you haven’t had sex with your partner for over a year, or if it only happens once/twice a year, why is that? I understand that kids, postpartum depression, and life stress can play a role, and temporarily halt your sex life, but some couples are having sex just twice in four years. I want to understand the other side of that.

Does time just slip away? You didn’t realize a year passed and you weren’t keeping track of your sex life? Do you worry about your partner cheating ever, or assume they’ll just deal with it? Do you believe things will eventually go back to normal, or does it feel like the end of the relationship is inevitable? If you’re consistently turning your partner away month after month, has attraction faded? Do you feel guilty about it? What’s happening in the relationship from your perspective?

Also for people in these situations, what are your thoughts on open marriages? I can understand not wanting to divorce, but is it realistic to expect your partner to also abstain from sex indefinitely? If you have a low libido, why do you want to have a partner with a high libido?

Also, why is there so much resistance to seeing a sex therapist—because the problem is “not that bad”? If you’ve been to one, I’d love to hear about your experience as well. Again, just trying to understand the mindset and emotions behind why this happens.