The idea would not be to uphold or relativize contraception, which is a gravely sinful practice, but to recognize that their life together consists of more than the perversion of sex.
Nevertheless (in agreement, I think, with you), I would not want any such venture to "find the good" in these defective relationships, in a Vatican document. It's the kind of thing you could explain in a novel, but it will be exploited as a sentimental pat-on-the-head "attaboy" if it's put into a teaching document.
That analogy fails. The sodomite relationship, is, at its essence, in defiance of God and nature (1605 Holy Scripture affirms that man and woman were created for one another). The "contraceptors" are engaging in a sinful act within the natural relationship established by God. If they resolve to cease engaging in the act, the problem goes away.