I think Bill Nye once described sex as basically the proverbial carrot-on-a-stick to get organisms to reproduce. I doubt he would say God has anything to do with it, but it sounds legit.
But to take a pragmatic approach: The unitive aspect, IMO, is mostly in the sense that a it provides the organism with a stable source of sexual pleasure. That is, you don't need to run around chasing after mates every year like animals, because you are united in a stable relationship. Stable relationships tend to imply co-dependence and mutual obedience, which are things that help us in our struggle for holiness. In that roundabout sense it is important to marriage, for those who are called to that theotic (?) path.
I don't think it needs to be theologized beyond that. Relationships are all about choices and sex is not a bare necessity, as any impotent marriage can show you. Some seem to turn sex into some mystical communion with eternity or something, and that's just bizarre to me. That's not to take a low view of sex; it is what it is. It is a bodily function to be used in moderation, in a specific context, for specific purposes, just like eating and sleeping.
I would say the primary point of sex is reproduction, and unity is a natural part of it. I don't think the two can be rightly separated.
Those are my random musings on the subject.