Mercutio cleverly turns Romeo’s remark, “Under love’s heavy burden do I sink,” into a sexual innuendo. If Romeo should sink "it" in, then he would “burden love” with his body weight, which would be “too great oppression (pressing down) for a tender thing” like Rosaline. 

But Mercutio’s choice of words allows for another interpretation. To burden means to charge someone with a crime, and oppression is the crime of forcible sex; so Mercutio’s statement could also mean that if Romeo forcibly has sex with young tender Rosaline, he might be charged with a crime. Romeo, however, is completely oblivious to Mercutio’s innuendo and continues to complain about how badly love is treating him.