Here:
“When a woman breaks up with a man, she usually wants every remnant of him removed from her life. A new study suggests that, try as she might, there may be one last piece of him that she’s stuck with for good: his DNA. A study from Australia has managed to prove that fly offspring are able to resemble a mother’s previous sexual partner, even when conceived with their father’s sperm.”
“Male microchimerism in the human female brain“.
“Male microchimerism was not infrequent in women without sons. Besides known pregnancies, other possible sources of male microchimerism include unrecognized spontaneous abortion, vanished male twin, an older brother transferred by the maternal circulation, or sexual intercourse.” [Emphasis in bold added.]
Another possible source of foreign DNA besides semen is saliva. Some exchange of saliva seems very likely between lovers, especially from “French kissing”:
Swallowing semen might also transfer DNA fragments. It has been claimed for some time that DNA in food can be transferred into tissues:
Dominant homosexual men sometimes crow that making another man swallow their semen marks him as their inferior forever. Scientifically, they may have a point.