Why? Because all people deserve the same rights, responsibilities and privileges. The right to be in love with someone and to share your life with them and to have the security of that relationship can't be reserved only for folks who fit one particular sexual orientation.
I know that same-sex marriage is a hot-button, wedge issue. And folks who are commited to following Jesus have disagreements about what the Bible says to and about today's LBGT folks. I think that the Bible passages that have been used to support the exclusion of LBGT folk from church leadership, adoption, marriage etc. are interpreted wrongly to do that. Others disagree with my opinion.
When we come to valid and significant scriptural disagreements about these issues, I think we have to measure actions by the overall message of the Bible and by what Jesus identifies as the two most important (and inseperable) rules:
Overall message: God creates -- we mess up -- God reconciles -- we mess up . . . .
God is about reconciling -- building relationship -- we are the ones who seem to always tear things down - and God responds with more reconcilitation.
The two big rules: Love God. Love Everyone.
The church's actions as it relates to LBGT folk must be in step with the message and with the rules.
I believe everyone should have the right to tie the knot.