I am a Catholic and have been asked to be best man for a good friend of mine. He and his fiancée, both Protestants, plan on not having children. They are clear about that. I do not believe either is infertile; they merely do not wish to have children. Is it wrong for me to be a part of their wedding, including acting as a witness to it?
If this Christian couple is absolutely closed to the possibility of life, then their marriage might be presumptively invalid. If that is the case, there may be just reason to attend the wedding as a guest (see link below for information), but I cannot recommend participating in the wedding as an honor attendant.