I want to create a world where tying yourself to a single partner would be considered irresponsible because it is potentially deadly. The more often two people come together and exchange DNA through kissing etc, the more likely for them to develop illnesses due to repeated contact with each others DNA. Naturally, the people would be either more social or more introverted. This would be because you either want to limit exposure to other people completely or you wish to have more variety in your life, thus lowering your risks overall. Contact could be limited by increased layers of clothing and more vigorous cleaning regimes, but I'm not sure how the biology would work.
Another risk is that repeated exposure to the DNA of another person could be used as a weapon - say a disgruntled ex repeatedly exposing their partner to their DNA to cause them to develop diseases and die. Also, if two people wanted to have a baby together specifically, how would they do so without risking either person's life if they are initially unsuccessful. Furthermore, if they are successful, how could I plausibly explain some form of resistance between parent and child to prevent children from being passed around or rotated through families to prevent people from becoming sick. After all, babies often drool, spreading their DNA rather effectively all over whatever they come into contact with.
What would be a way to make this repeated exposure dangerous without using too much handwavium and how would resistance to children/parents be explained.