Threesome with 2 girls 1 man

Hello. My hubby and I plan to have a threesome with another stranger girl. At first I agreed but the back of my mind I am scared of the possible outcomes of our lust. What if I can't handle seeing him f*cking another girl, what if the girl has STD or other infection. But I wanted to explore more about our sex life. Please give me some advice if I should continue this fantasy or not. He said if I don't want to do it, it's okay he will respect it and if I do it he will do that out of lust only if I want to stop it will. The decision is mine. Also I was the one who suggested this idea. Thanks.