l was once submissive to my friend's husband
How do you feel about being the sub for your friend's husband?
I find it quite natural because I've been accustomed to doing things for him.
You should never betray your friend's trust by being with their partner. Instead, you should find another person that is single and into the same things that you are into.
Nah, it's fine. I'm happy with my life.
As long as your friend knows and their trust isn't broken you might be okay. I personally don't mix friendship with sex but some people are capable of doing so without bad feelings. as long as you keep an open communication between you all then hopefully it works out for you.