Adam Clarke on Romans 6:19

I speak after the manner of men - This phrase is often used by the Greek writers to signify what was easy to be comprehended; what was ad captum vulgi, level with common understandings, delivered in a popular style; what was different from the high flights of the poets, and the studied sublime obscurity of the philosophers.

Because of the infirmity of your flesh - As if he had said: I make use of metaphors and figures connected with well-known natural things; with your trades and situation in life; because of your inexperience in heavenly things, of which ye are only just beginning to know the nature and the names.

Servants to uncleanness, etc. - These different expressions show how deeply immersed in and enslaved by sin these Gentiles were before their conversion to Christianity. Several of the particulars are given in the first chapter of this epistle.