As for marriage itself, I have only opinions. The Bible does not offer us a ton of information on eternity, unless there's something I'm missing, but in this life I agree it's about creating a stable, cohesive environment for family-building. Secondarily, I think it's about the human need for companionship -- "A help meet..." Maybe that's even before family-building, since it's the first reason given for creating Eve.
In eternity...
I think our genders remain. Our bodies will be changed to something that cannot die, but not necessarily genderless. Jesus was still male after His resurrection, unless the writers left something out.
God promises to "restore" all things. What does that mean? He also promises to make all things new. Does that mean scrapping the old and re-creating everything into something different? My opinion, it's a combination of restoration and redoing.
Because God is good, and because I believe He wants us to enjoy life, I believe sensual enjoyment will be part of the experience. Sight, touch, taste, smell, etc., all bring pleasure, and I don't see why that would change. As for sex itself, I have only speculation (as do any of us). My speculation includes multiple possibilities.
1. There is sex, somehow, some way, since our bodies will be as they are now, only they will not decay... and maybe we'll have what we now would call "super powers"
2. Or, there is something better than sex. Peter Kreeft said something like this: As we grow up, our genitals start off as waste-excretion zones, then "graduate" to a higher purpose when we become adults -- without losing their original purpose. Heaven is the ultimate graduation. So, whatever God plans
may make sex look like peeing.
3. I had dreams about my late wife. Many, actually, but three stand out. They were the first three dreams I had of her after she died. They seemed to be part of a series, separated by a few months each, and they both were very real, in very believable environments. I'll spare you the eye-glazing details, but I have some reason to believe these were from God and not normal dreams. If they were from God, my honey responded to me like someone who was still in love with me, not a generic, asexual, spiritual being. No, we did not have sex, but there were other expressions of love and affection that, say, two male friends would not do to one another.
All speculation, yes. But that's all we can do is speculate, based on what little info we have. And, for right now, not having that intimacy in this life for most of my life (~70% of my adult life), I have to hope that I'll have some form of it when this life transforms.