First of all I am a progressive Catholic. I believe in love. I believe that people should have a right to love whomever they want to. Gay or not. It is not my place to tell people how to live their life.
Welcome to the forums.
Love is Good. But love is not sexual jollies at the expense of another person, even if that other person is willing. True Love is selflessly wanting the Best for another. Without getting into a long discussion about the nature of the "Best", it should be plainly obvious that the "Best" is not sexual self-gratification (gay or otherwise) because it involves both selfishness and a relationship that is not oriented to the benefit of the other person, children, or society. There is no "Best" to be found there. Regardless of how you want to characterize a homosexual coupling, it is plainly not about "love" because it cannot be freed of its selfish aspects.
The theme of "love" is used by gay activists to confuse people precisely because so many do not understand true love. A result of Hollywood/media overload, I suspect.
Regarding you last sentence, you are called to make such judgments every day. You see a man beating a woman. Do you turn away? Do you say "The woman could leave if she wanted, but who am I to tell her how to live her life?" You see someone preparing to leap off the Golden Gate. Do you say "Whatever. It is not my place to tell people how to live their life" and walk away? Laws are on the books for the purpose of circumscribing behavior and ordering society. For example, brothers and sisters cannot/should not have conjugal relations. Same for parent/child and first cousins. Those laws are there for a purpose. They should not be overturned because of a false notion of "love".
Look at the flip side. By supporting "gay marriage" you are doing precisely what you are claiming you want to avoid. You are telling others they MUST support it. Taxpayers MUST underwrite it. Children MUST live under it. You are telling those that see it as an abomination that they MUST accept it. Freedom of Religion is gone. Freedom of Conscience is gone. That is not progressive, but regressive.