Just about every medical professional would advise against marrying a first cousin; yes, there are many thingks that are allowed under daas torah but that doesn’t mean you are obligated to do them. The torah allows a number of punishments including stoning for a variety of “crimes’ (putting aside whether or not we have a Sanhedrim), allows for taking multiple wives (putting aside Rav Gershhon’s thoughts on the issue), etc. etc. Just because something is allowed by either Torah or civil law doesn’t mean its a good idea in the context of modern society and norms. I’m sure Joseph may feel its a great idea (and he can find a rav to give him a heter) to marry his 15 YO first cousin but that doesn’t mean 99.9 percent of yidden today would do so.
