How deal with her worries about STDs

Testing is key!! If you are active with multiple partners this needs to be done on a regular basis. Plus the partners need to be telling you the truth that they are clean. Realize a lot will lie about this knowing they are infected! So you have to be very aware of who you are with.
 
Our bulls test regularly
Having said that, you must also know and trust your partners as best you can. If you do not, then a condom is a must. Aside from her six regulars, my wife always uses a condom, especially if it's a first time or a one night stand, which does happen a handful of times each year. For that reason she always keeps some with her when we go out to a bar or club and I keep some in reserve in case she forgets. If the spontaneity of a one night stand is to be indulged and enjoyed, condoms are a must in my view.