Is it hooking up with someone and putting your mouth on unspeakable places?

No.  Lots of people have one night stands.  They wake up in the morning and realize that they have no interest on spending the rest of their life with the person sleeping (or pretending to be asleep) next to them.  All they have to look forward to is the walk of shame.  So, since they are probably hung over, they gather up enough energy to get their belongings, leave and head home.

Is it when they move in together?

No.  Lots of people in this situation end up cheating.  They just need enough energy to pack their belongings, rent a truck, leave and move in with their new "Lover."

Is it when they get married?

No.  Lots of people in this situation end up in divorce.  They just need enough energy to find a new place to live, pack their belongings, rent a truck, and leave.

It seems like we have hit a wall in trying to identify true love.  Maybe if we define the opposite of love we can make headway.

Is the opposite of love when you have enough energy to gather up you belongings, find a new place to live and rent a truck?

No.  Lots of people have just left everything behind and never looked back.  Very little energy required.

The only recurring theme for the opposite of love is having a place to go.  Therefore, True Love is when you have no where else to go and so... you just stay.