One more reason why promiscuity is not such a great idea - microchimerism:
Microchimerism is the presence of a small number of cells that
originate from another individual and are therefore genetically distinct
from the cells of the host individual. This phenomenon may be related
to certain types of autoimmune diseases; however, the mechanisms responsible for this relationship are unclear.
While the Wiki article goes on and on about maternal microchimerism, it does mention at the end of the section that
It is hypothesized that unprotected intercourse with ejaculation may be another source of microchimerism.
Something to think about?