If you don't believe in natural rights and accept their existence then I will just exit the conversation right now.
And you are right. I am a theist. Catholic. I also belong to an society built of objective thinking. And I am considering all aspects of marriage. And I don't see how the "ultimate goal" per-say is procreating. BUT. I will stoop to your level (provided you accept natural rights), and debate with you as if the ultimate goal and our drive is for the prolonging of humans. Over population will end the human race. And homosexual marriage helps because they don't procreate. And the more homosexuals that exist in the world, the more people not procreating at the highest rate ever known to man. It's a HUGE issue right now. So simply for your shallow reasoning of prolonging the human race rather than any aspects of love, one should support domestic partnership.
And as for them adopting: Why on earth would you not allow two people who love each other to have a kid? Would you rather them live as orphans? You need to understand that many homosexuals would take a child as such a lavish gift that they could potentially treat them incredibly well.
And again, sinking towards your incredibly shallow state of mind about marriage: Doesn't homosexuals having more rights help the economy? If the work place doesn't discriminate against sexual preference, homosexuals can start acquiring higher paying jobs and purchase expensive items, and further help and economic crisis.
So there are your immoral yet statistical points of why homosexuals should have equal rights.