No. I don't think they do.
I know this league has groupies throwing themselves at these players.
And I'm sure many of them take the groupies up on it.
But I don't think Duncan is that kind of man.
When he was single who knows, that's his business.
But as a married (happily from all reports) man and a father to 2 young children I do not believe he fools around.
Duncan has presented a certain kind of character his entire career. Loyal. Committed to his team. His game. His charities.
Fooling around on his wife goes against everything he's about.
I don't know him, this is only my opinion. But I've always believed that what we see is who he is. On and off the court.
There's a wonderful article from some years ago where Amy talked about what happened when Duncan was drafted. That she didn't want to be an "NBA wife", knowing what could come along with that. She was ready to break it off with him and let him go his own way. She said Duncan proved to her that he wasn't going to be that way, he wasn't going to do "those things."
Duncan called her from every city he landed in his rookie year. Sometimes several times a day. He let her know his love was true and that nothing, not even NBA groupies and the NBA "lifestyle" was going to change him or his feelings for her.
I believe their marriage is "real."
I do not believe for a minute that he would fool around with some flunky and risk that.