What do you mean "doctors have discovered"? That the sperms are foreign substances to the woman's body has been known for quite some time.
You're half right. The sperms leave parts of itself in the membrane in the womb, so they will be recognized in subsequent times. The first new* sperms will have to fight off this earlier remains (sort of), and then they will be remembered the next times instead.
I seriously doubt that this can cause cancer, as it's not a confusion as much as it is a replacement. It's not like stopping eating nuts and switching to berries will cause cancer, is it?
*The first man could have died, and the woman has remarried, ya know.
The average number of sexual partners is 5-6 for women and 6-8 for men. You have to be a bit over that before you are called "promiscuous", I'd say.