I think it would depend on your relationship with the couple in question.
First off, I would assume that a couple that could afford IVF would not be in the same vulnerable position as a young single mom, and would be less in need of baby items.
If the mom of the baby was my daughter or close relative, I would go no question. I would want a relationship with my daughter, grandchild, etc no matter what.
Ask yourself how you would feel facing the baby as an adult. Can you imagine saying “I did not go to the event to celebrate your birth, etc, because I disapproved of how you were conceived and brought into this world?” If not a very close friend or close family member, you might just send a gift for the BABY and not go.
At the end of the day, other people have different religious beliefs than you. Not everyone believes pre-marital sex is wrong, not everyone believes gay marriage is wrong, not everyone believes IVF is wrong. I don’t expect people to have the same beliefs I do about a variety of things, yet I still celebrate their various life events. I am not going to limit my friendships and relationships to those who are following strict Catholic guidelines. Not even my Catholic friends follow all Catholic beliefs (like birth control). People on this board are much more hung up on these things than the Catholics I know in my life.
Even the Catholic hospital that is my employer provides benefits to spouses of same gender marriages and welcomes their children, same as any other employee.
And what is it about being gay lifestyle that makes people half shun the person in question more than other departures from scripture. For example, there appear to be about 100 directives in the Bible to serve the poor, but I don’t see threads like "Uncle George does nothing to help the poor, and is kind of a judgmental jacka** about poor people, goes to a Protestant church, but does not remember the Sabbath day and keep it holy, can we still invite him to Christmas dinner/attend his wedding/go to his wife’s baby shower/let our kids be around him under our supervision even though he is a poor example. I get why there is a belief that same gender marriage is not God’s plan for family, but not why this seem to trump many other diversions of religious disagreement.