is Mercutio's idea of love only associated with sex?

Yes, Mercutio's vision of love is the prelude and attainment of sex. Mercutio sees love as an outlet for sexual aggression.

".... if love be rough with you, be rough with love/Price love for pricking, and you beat love down”.

Mercutio's language is direct and controversial. He speaks of women as objects of desire, nothing more. Shakespeare's use of wordplay in Mercutio's dialog is unequaled..... and the innuendos are no accident.... they're exactly what you believe them to be. An example of this is found in the following lines....

If love be blind, love cannot hit the mark.
Now will he sit under a medlar tree
And wish his mistress were that kind of fruit
As maids call medlars when they laugh alone.
O Romeo, that she were, O that she were
An open-arse and thou a poperin pear!