Some find it to be morally wrong.
To grow up to be well-balanced adults, children need role models of both sexes. Boys without fathers under-achieve, especially since there are now fewer male teachers in primary schools.
We are a 'Christian' country - even if few go to church, our values remain based on Christian teaching. Two parents are axiomatic - 'Honor thy father and mother' invokes the Fifth Commandment.
Children raised by gay parents are offered only one partnership model and are therefore (some argue) more likely to be gay.
If Roman Catholic adoption agencies close rather than allow gay couples to adopt, the number of adopted children will decline, leaving more in the unsatisfactory care system.
Some areas of life cannot be legislated for and must be left to individual conscience. A sufficiently large minority simply find gay parenting 'wrong'; the practice therefore should not be enforced on all.