First of all, in all the human the history of unfortunate comparisons, this has to make the top ten. Lesbianism/homosexuality is just like being a Methodist? That's rather insulting to our separated brothers and sisters in Christ, don't you think?
Let’s take another case – the parallel isn’t perfect, but it may be helpful. If your Methodist friend asked you to be godparent to her first child, you wouldn’t be able to accept. You couldn’t commit yourself to insuring that the child be raised Methodist without implicitly, at least, admitting the validity of the Methodist religion. But as a Catholic, you can’t do that without renouncing your own Catholic faith. So, you would try to explain this to your friend. And in order to show that you still care about her as a friend, you might very well agree to come to the post-baptism party.
Attending the wedding reception of an openly gay friend or relative is similar. By doing so, you can support the person while making it clear that you don’t support that person’s every decision – in this case, the decision to stay Methodist, or to actively live a homosexual lifestyle. Because of the context created by your conversation with your friend, your attendance at the reception would not be a celebration of their Methodism or Lesbianism, but an expression of your care for them as a human being and a friend.
Secondly, a Catholic may, in fact, ordinarily attend a Methodist baptism, especially if the friends in question are lifelong Methodists (e.g., they are not former Catholics who left the Church and became Methodists). While the Catholic may not act as a godparent or take any active role, he or she is perfectly able in the spirit of ecumenism to attend a baptism that is putatively a valid Christian baptism (since ordinarily Methodists do use a valid form of baptism). There is no question of "only" attending the party afterward, since the party afterward is to celebrate the baptism and the baptism is something a Catholic may, under ordinary circumstances, attend!
The only time the questions of prudence Fr. Bartunek mentions might come in is in a situation where a Catholic who has married a Methodist outside of the Church is having his child baptized in the Methodist church. In such a circumstance, the close Catholic family members of this person might have to weigh whether or not their attendance at either the baptism itself or the party afterward will be helpful in drawing the lapsed Catholic back toward the Church, or if their attendance at either might be a barrier to that return (for instance, if the Catholic family member thinks their attendance is a proof of religious indifferentism on their parts). But that knotty, complex situation isn't even close to what Fr. Bartunek describes--he is talking about a Catholic person and his or her Methodist friends, and in that circumstance there would be no particular reason for the Catholics to avoid either the baptism or the celebration of that putatively valid baptism afterward--for, indeed, the child has been baptized, and while we may mourn our sad divisions in Christianity, who can help but rejoice when a baby becomes a child of God, and an heir to the Kingdom?
But, thirdly, and this is the critical point--in no way is this situation even remotely analogous to the difficult and painful question of how Catholics must act when invited to celebrate a gay union, "marriage" or not. Catholic apologist Michelle Arnold has said, "Since the Church has spoken so strongly against "same-sex marriage," I cannot recommend attending or celebrating "same-sex weddings" under any circumstances." (Link in her original quote.) Arnold's opinion is shared by many others, and I know I've heard or read other priests give a similar opinion (and if any Catholic priests are reading this blog, and would like to weigh in on this issue, I'd be very appreciative!).
Why shouldn't Catholics attend gay "wedding" ceremonies, or show up for the reception afterward? Because Catholics do not believe that same-sex couples can "marry" in any real sense of the word. Showing up for the celebration afterward would, in effect, be showing up to celebrate a lie, from the Catholic perspective; it would be showing up to celebrate the couple's commitment to continuing their practice of engaging in gravely sinful sexual behavior, behavior which, objectively speaking, may be endangering their immortal souls, cutting them off from the life of grace, and wreaking spiritual ruin within the depths of their beings.
Clearly, this isn't something Catholics can celebrate. Equally clearly, it is of an order far different than the question of whether Catholics can celebrate a valid, if Protestant, baptism when the parents of the child are Protestants! It is much more like the question of whether a Catholic may ever attend or celebrate any other sort of invalid wedding, with this one great difference: the marriage of a man and a woman might be putatively valid in many cases, but the "marriage" of two men or two women is never putatively valid--it is invalid by definition, as far as the Church is concerned.
I find it deeply unsettling that a Catholic priest would not appear to see any significant difference between these two hypothetical situations, when in fact the difference is astoundingly clear. Perhaps Father Bartunek merely wrote carelessly, without greatly considering the implications of what he was writing. Still, once again I find myself wondering just what, exactly, the Legion's formation of priests consists of, and how it is that a lay Catholic apologist like Michelle Arnold seems to have a much clearer grasp of what is at stake for Catholics in regard to giving any sort of approval or condoning of gay "marriage" than a Legion priest, apparently, does.