Apart from deNiro’s TDS, I would imagine he has been Tom Catting for most of their time together. The ability to ‘get some action’ is always there; rich, appealing to many women, enjoys the constant attention of others outside the family, is away from the family home most anytime he wants to be, and for as long as he wills it so.
I would guess being married to someone famous means you have to accept their lifestyle too. Flirting, or pretending to, is often seen as a part of doing business in the entertainment world. They may not be as monogamous as the public pretends to expect.
Mick Jagger is said to have told some of his wives that he would keep on keepin’ on when he was on tour.
In other words, take it or leave it.
If I remember....DeNiro was caught up in a prostitution ring in Europe some years back.
His father was a bisexual, which cannot have helped his mental stability. On top of all that he seems to have a double-digit IQ.