They are both the results of sex. No sex, no STD's or unwanted pregnancies. You are also begging the question of whether adulterous sex or premarital sex are responsible. Please note; I said nothing about homosexual sex. That's your insertion. Though to be honest, since gay marriage is legal (and finding a permanent partner to whom one is committed and 'true' was possible before gay marriage was legal) the same points ...well, not the unwanted pregnancies, but CERTAINLY the STD's....are still a problem if one commits adultery or has premarital sex, or sex with multiple partners. Gay or straight, the principle is the same.
Define 'lots.'
The folks who engage in what they call 'open' marriages, or who are 'swingers,' (or whatever they call it) are in an extreme minority. And I know of none who have celebrated fifty years of marriage. Do you?
Actually, YOU are the one using your gender to make very obviously male-centered arguments. I can't think of a single woman who would be as blithe about making the points you are attempting to make here, because women DO bear the brunt of the negative consequences of pre-marital and extra-marital sex. They have to. They can't get out of it. Men...if they CHOOSE, can stick around and help, but they don't have to. They CAN walk away. I've borne five children. You don't get out of being pregnant by saying 'I've changed my mind" and walking out of the door.
Even if one decides to get an abortion, bub, that is NOT a trip to a day spa.