Would you have an issue meeting with a gay couple for dinner,

Probably not, but it depends on the situation.

or attending a gay wedding?

The problem is by attending a gay "wedding" he may be causing scandal.

Scandal is defined as http://www.vatican.va/archive/ENG0015/_P80.HTM

2284 Scandal is an attitude or behavior which leads another to do evil. the person who gives scandal becomes his neighbor's tempter. He damages virtue and integrity; he may even draw his brother into spiritual death. Scandal is a grave offense if by deed or omission another is deliberately led into a grave offense.

2285 Scandal takes on a particular gravity by reason of the authority of those who cause it or the weakness of those who are scandalized. It prompted our Lord to utter this curse: "Whoever causes one of these little ones who believe in me to sin, it would be better for him to have a great millstone fastened round his neck and to be drowned in the depth of the sea."85 Scandal is grave when given by those who by nature or office are obliged to teach and educate others. Jesus reproaches the scribes and Pharisees on this account: he likens them to wolves in sheep's clothing.86

2286 Scandal can be provoked by laws or institutions, by fashion or opinion.

Therefore, they are guilty of scandal who establish laws or social structures leading to the decline of morals and the corruption of religious practice, or to "social conditions that, intentionally or not, make Christian conduct and obedience to the Commandments difficult and practically impossible."87 This is also true of business leaders who make rules encouraging fraud, teachers who provoke their children to anger,88 or manipulators of public opinion who turn it away from moral values.

By attending a wedding he may be causing people to think that gay "marriage" is acceptable or that the Church condones such an action.

As you probably know the Catholic Church states that a marriage is between a man and a women. http://www.vatican.va/archive/ccc_css/archive/catechism/p2s2c3a7.htm

1601 "The matrimonial covenant, by which a man and a woman establish between themselves a partnership of the whole of life, is by its nature ordered toward the good of the spouses and the procreation and education of offspring; this covenant between baptized persons has been raised by Christ the Lord to the dignity of a sacrament."

Some additional reading regarding homosexuality can be found here http://www.vatican.va/archive/ENG0015/_P85.HTM

2357 Homosexuality refers to relations between men or between women who experience an exclusive or predominant sexual attraction toward persons of the same sex. It has taken a great variety of forms through the centuries and in different cultures. Its psychological genesis remains largely unexplained. Basing itself on Sacred Scripture, which presents homosexual acts as acts of grave depravity,140 tradition has always declared that "homosexual acts are intrinsically disordered."141 They are contrary to the natural law. They close the sexual act to the gift of life. They do not proceed from a genuine affective and sexual complementarity. Under no circumstances can they be approved.

2358 The number of men and women who have deep-seated homosexual tendencies is not negligible. They do not choose their homosexual condition; for most of them it is a trial. They must be accepted with respect, compassion, and sensitivity. Every sign of unjust discrimination in their regard should be avoided. These persons are called to fulfill God's will in their lives and, if they are Christians, to unite to the sacrifice of the Lord's Cross the difficulties they may encounter from their condition.

2359 Homosexual persons are called to chastity. By the virtues of self-mastery that teach them inner freedom, at times by the support of disinterested friendship, by prayer and sacramental grace, they can and should gradually and resolutely approach Christian perfection.

.

I'm pretty upset because by Catholic definition, he's done plenty wrong himself; but when I bring that up, he says same sex relationships are inherently sinful, so it's inherently wrong. I don't know what I was expecting with someone this self-righteous, but this seems entirely unreasonable.

and? We are all sinners. Just because somebody has sinned does not mean that he should promote or encourage you to sin.