Birthday cake, I'd have no problem making that for them. A wedding cake is a different level due to the marriage ceremony aspect. As to the tree's above their wedding, I would honestly have to think about it. I feel it is somewhat different though. Unlike a cake it isn't for a one time benifit/enjoyment.
Would you build a shed/house that you knew without a shadow of doubt was going to be used for evil purposes? I'd hope you wouldn't. For me making a wedding cake would be along the same lines (though probably less extreme).
We are all sinners, including myself. The difference is I try to recognize my sin and not continually live in it. I understand gay marriage is socially acceptable now, but that doesn't mean I have to morally except it too.
To provide a service for someone isn't always an acceptance of someone or of their personal life. A wedding cake is different as I explained before.