Rather than 'threatening' marriage, the marriage of gay or trans people is an extension of the potential beauty and grace of marriage to groups of people to whom it was previously denied.
There is no confusion of the various types of love. You have mentioned agape and eros. These loves may exist (or not) in relationships of all types.
Procreation has always been optional to an extent - as evidenced by the fact that the Church has always been prepared to solemnise the marriages of those who cannot have children (and No, it does not do this in the hope of miraculous births. It does so because it accepts that other purposes of marriage are also praiseworthy).