Anyway, here is the section I want to highlight:
In philosophical and theological debate, these distinctions [between agape and eros] have often been radicalized to the point of establishing a clear antithesis between them: descending, oblative love—agape—would be typically Christian, while on the other hand ascending, possessive or covetous love —eros—would be typical of non-Christian, and particularly Greek culture. Were this antithesis to be taken to extremes, the essence of Christianity would be detached from the vital relations fundamental to human existence, and would become a world apart, admirable perhaps, but decisively cut off from the complex fabric of human life. Yet eros and agape—ascending love and descending love—can never be completely separated. The more the two, in their different aspects, find a proper unity in the one reality of love, the more the true nature of love in general is realized.As someone remarked soon after this came out, Benedict just called out CS Lewis and said, "you're wrong."
To put it another way, while eros is not necessarily agape it can and should be. And to draw an exclusionary line between them would be to detach Christianity "from the vital relations fundamental to human existence." And the most important of these fundamental relations is marriage.
I've said this over and over—and I'll stop for a while after this time, honest—if I'm married and the eros has gone out of my marriage then I am obliged to do everything I can to rekindle it. This isn't voluntary. Finding and maintaining a proper unity between agape and eros is what I promised in a sacred vow to God in the presence of God's priest and in front of the community. If I don't do everything I can* I have betrayed that vow and betrayed my spouse in a way that is at least as bad, and potentially worse, than if I had an affair.
* Which isn't to say that there might not be physical reasons preventing me from succeeding. In that case, the to-have-and-to-hold vow is succeeded by the in-sickness-and-in-health vow.