Well, if you have a husband or a boyfriend, it has been proven that you are most likely to conceive when cheating on him. So, if you want to saddle the poor shlub with someone else's child then have at it.
I have heard of this sperm battleground thing. Not sure how true it is. The less healthy sperm actually act as a blockade which fresh sperm have to get around. And there are other sperm whose job is supposedly to roam around looking for enemy sperm to mutilate.
As I said, not sure if it's true. I've seen reports that say it is and reports that say it isn't.
Hopefully someone does know more details. I've always been interested in this concept.