How do non-trinitarians respond to the argument that God is love necessitates the eternity of the Son? A common argument for the Trinity (or at least the eternity of The Son) is that God is love, and thus there must have been an object that could be loved by him throughout all eternity. The Trinity's idea of fellowship within unity seems an attempt to deal with this issue.

If Jesus the Son of God had a beginning, then it follows that before the existence of the Son, God being solitary, would have no object towards which to express his love. Without having anything to love, it seems God could not rightly be called “love” or possess love from all eternity.

How do non-trinitarian Christian's respond to such an argument?

Perhaps they would argue that whether the Father loves himself, or the Father loves the Son, God is still loving himself since both the Father and the Son are God, but I am interested in seeing what others say.
The problem with this argument is that it's not based on Scripture. It's based on the unsupported assertion that "there must have been an object that could be loved by him throughout all eternity."

Jehovah's Witnesses are non-trinitarian, but they don't reason on things that the Bible doesn't comment on, such as the details of what God did before the beginning of creation. They do agree, however, that God is love (1 John 4:8) and that his identity and personality does not change. (James 1:17; Malachi 3:6;  Isaiah 43:10; 46:4)