I can't speak for others on this, but I have always been very open about who I have 'done things with' and still see from time to time as friends with my subsequent girlfriends.

Communication and transparency. I am married and my wife knows, has met and is friendly with all of my ex fwb that i am still in touch with. If i became single and my new girlfriend couldn't handle that i am friends with someone I have previously had sex with, then they are not the right person for me. I am not going to walk away from a totally secure and loving friendship with people I have known for over 30 years in some cases just because someone I met many years later it feels all insecure.