Spiritually speaking, is it possible to be in favour of gay marriage, but opposed to the couples having kids?

It is possible and logical but I strongly disagree. Most animals in nature only have one parent - the mother. Fathers hang around only in a minority of cases. In humans mothers have only survived childbirth consistantly for about 60 years so single fathers were very much the norm before that. In humans a child simply needs to have family members of both sexes - it does not need to be the parents. I am sure that there are some children who would pick on the child of a gay couple but unless we are also going to ban overweight people, unattractive people, religious people, people with disabilities, poor people etc etc from having children bullies will always find a target.