The unitive aspect is essentially about the physical union itself, providing the possibility of procreation by the natural principles within the completed system of reproduction. (This would be opposed by contraception, but also more obviously by IVF.)
Paying the “marital debt” as it is called does not need to be psychologically unitive in order to be morally good, though of course they are related and feed off of each other.
The meaning of “procreative” is generally understood (by Catholics) as meaning “of a form ordered to procreation”. Thus, intercourse between the infertile remains “procreative”, but intercourse using a condom is not, and neither is intercourse in conjunction with the contraceptive pill.
Unitive is often poorly understood - and I have on occasion gone looking for an explanation, and found very little. I have come across your description (“the physical union itself”) and also descriptions based on the presence of various other elements or effects of sexual intercourse, such as sharing/bonding/attachment, etc.
The difficulty I have with your explanation is that it does not make very much of a distinction between procreative and unitive. For example - is there any procreative act that is not Unitive? IVF - as a set of acts - certainly involves no unitive (as per your description) element. Is IVF the only way to remove the unitive from what remains procreative?
So, I am inclined to broaden the view of the unitive aspects to encompass more dimensions of the relationship of man and wife, as follows: “The unitive aspect of marriage has both spiritual and sexual components. In both, the man and woman are joined together into one flesh through love and total giving of oneself to the other. The total giving is not just physical. Through the Sacrament of Marriage, the couple is spiritually united into one flesh, thus, totally committing themselves to one another. Through the physical, they are also joining together into one flesh; thus, the physical aspect is an expression of that union.”
I suggest that: Acts such as the sexual intercourse of unmarried people & selfish acts are not truly unitive regardless of whether they are procreative.
