The idea of relations "that were unnatural" is a euphemism for immoral sexuality. AT: "started practicing sexuality in a way God did not design" (See: [[rc://en/ta/man/translate/figs-euphemism]])
Here "natural relations" is a euphemism for sexual relationships. AT: "many men stopped having natural sexual desire for women" (See: [[rc://en/ta/man/translate/figs-euphemism]])
Here "in themselves" refers to "in their bodies." You can translate this in an active form. AT: "God has punished them by sicknesses in their bodies" (See: [[rc://en/ta/man/translate/figs-activepassive]])