Let's say same sex marriage is "legalized". Will private institutions (like churches or adoption agencies) be forced to service same sex couples? For example, Could a church be legally compelled to marry a gay couple? Would a catholic adoption service be obligated by law to work with adoptive gay couples?

I'd say we're only talking about civil marriage recognized by the state here. A it stands now, gay people CAN already get married by the churches that recognize that right, it is just not state sanctioned. If it is legalized, religious institutions can continue to provide marriages as they see fit, thy are just separate from state recognition. They should not be compelled to perform a religious ritual.

On the second point, yeah, they damn well should have to let gay couples adopt. If they have a problem with it, they should get out of the adoption business. I feel the same about religious doctors and pharmacists. If a girl comes for the morning after pill, you can't decide you're not going to give it because of a personal preference. Not your IMO.