It seems to met that you can express love and a desire to be an ongoing part of that person's life, without pretending to "celebrate" the breaking of God's law.
A person who loves YOU, would not insist that you violate your conscience. Public approval is one way of participating in the sin. It makes one an accessory or accomplice.
You don't have to lecture them on moral law as long as they know, calmly and simply where you stand. That doesn't mean you stop showing them love. Inviting them over for dinner, for instance, is a way to express care for them as people, without publically affirming their irregular union.
There is no need for religion as grounds to justify not attending or condoning such a relationship. Merely finding the conduct of homosexuals wrong ON ANY GROUNDS (religious OR OTHERWISE) is enough grounds.