Mostly Christians get offended because marriage was defined by the Church, the taken over and redefined by the gov, and then subverted into whatever anyone else wants it to be. Note that normally, it's not enough for gay people to have the same rights to stay and live together, per the same laws that married men and women do; its that they have to be a giant PITFA because they want the Church to change the definition of marriage to include them (and therefore imply that what they're doing is right, good, or otherwise endorsed by the Church). It's not exactly that we're against marriage being redefined to being man + man or woman + woman - it's that we're against it begin redefined to _anything_ but man + woman. If we change it for this, where does it end? Man + goat? 2 men + horse + cow? And really, I could care less if that's your thing - but if you jam your thumb up my ass trying to get something that someone else created and defined into whatever your lust of the week is, well, we have a problem.
That's the major problem. The tenet is "hate the sin, love the sinner", but listen - it's much easier said than done, especially in emotional situations. One of the core tenets of Christianity is the belief that mankind is flawed, and can never measure up to an objective moral code; therefore, saying that all Christians should be perfect is about as smart as saying that all people in a hospital shouldn't be sick. And the "Christians" that imply otherwise are out of the loop. Those people do more damage to the Church that anyone else I can think of. There's even a word for those people; they're called "Churchians". Give that a search some night.
As to your second question, if you care enough to find the answer, recommend "Cynic Sage or Son of God", by Boyd. Not the best work, but a very good one for understanding the answers to some of those questions, and it's a good read, even if you don't agree.