You're entitled to your opinion, of course, but it seems to me that making a cake for a celebration of sodomy is a remote participation in the evil of sodomy. It is certainly condoning sodomy and a sodomitic relationship.
It seems to me that it's something a Christian should not do. It seems to me that it's something that "falls short of the Glory of God".
YMMV.
If you knew it was for that, the right thing would be to decline.