Although matrimony is not likened to the Passion of Christ as regards pain (poenam), it is likened to it as regards the charity by which He suffered for the Church who was conjoined to Him in marriage.
[St. Thomas Aquinas, Super Sent. 4 d26 q2 a1 ad3, my translation. See also Summa Theologiae Supp.42.1ad3.]
All sacraments, of course, have to link up to the Passion of Christ; the objector notes that matrimony concerns the pleasant ,not the painful, so can't link up with the suffering of Christ. Aquinas responds by saying that matrimony shares with the suffering of Christ not pain but love.
