Depends on a variety of factors and in my experience there’s plenty of stds in serial monogamists because there’s the assumption that since you are dating they skip all the testing and don’t wear condoms. But I’ve also ran into woman who got their birth control thing down and they don’t want to do condoms and just assume they will have good luck but that risk is definitely there when you have a routine thing going. If you both aren’t on the same page going into it, it really takes at least one person in the relationship/friendship to suggest it be a hard rule. Met plenty of recently-out poly women that assume it’s all just free play but they had long been sleeping around and just never thought of it because it was short lived interactions that ended in someone being clingy and them dating or the other person being a fuckbuddy that wasn’t willing to treat them like a friend/ be seen with them.