I stumbled upon a lecture given by Peter Kreeft at Columbia University some years ago in which he addresses this very question. After a lengthy and excellent consideration of the merits of the question, he concludes:
I think there will probably be millions of more adequate ways to express love than the clumsy ecstasy of fitting two bodies together like pieces of a jigsaw puzzle. Even the most satisfying earthly intercourse between spouses cannot perfectly express all their love. If the possibility of intercourse in Heaven is not actualized, it is only for the same reason earthly lovers do not eat candy during intercourse: there is something much better to do. The question of intercourse in Heaven is like the child's question whether you can eat candy during intercourse: a funny question only from the adult's point of view. Candy is one of children's greatest pleasures; how can they conceive a pleasure so intense that it renders candy irrelevant? Only if you know both can you compare two things, and all those who have tasted both the delights of physical
intercourse with the earthly beloved and the delights of spiritual intercourse with God testify that there is simply no comparison.
This spiritual intercourse with God is the ecstasy hinted at in all earthly intercourse, physical or spiritual. It is the ultimate reason why sexual passion is so strong, so different from other passions, so heavy with suggestions of profound meanings that just elude our grasp. No mere practical needs account for it. No mere animal drive explains it. No animal falls in love, writes profound romantic poetry, or sees sex as a symbol of the ultimate meaning of life because no animal is made in the image of God. Human sexuality is that image, and human sexuality is a foretaste of that self-giving, that losing and finding the self, that oneness-in-manyness that is the heart of the life and joy of the Trinity. That is what we long for; that is why we tremble to stand outside ourselves in the other, to give our whole selves, body and soul: because we are images of God the sexual being. We love the other sex because God loves God.
The entire article is well worth a read.