After twenty-plus years of marriage, sex can expand to include many things, including masturbation...
When my wife was in the later stages of pregnancy, intercourse became less advisable, and she did not feel like oral sex as much as I wanted some kind of sexual activity.
One afternoon I was horny, and she didn't feel very sexy, and she said, 'Why don't I just jack you off?'
That was fine with me, and over time we became more open about either she masterbating me or my beating off myself, as the desire struck me.
Today, while we still have good sex on a regular basis (two or three times per week), I beat off once, twice, or three times per day. I work at home, and often don't need to get dressed, so as I sleep nude, I ofen don't bother with clothes until mid-morning.
When the mood strikes me, I beat off. If my wife is around, she will give me some strokes or comment on my technique. She makes suggestions about stimulation, and will sometimes massage my prostate while I masterbate.
Regardless if she joins in, watches, or provides encouragement, it is always better when she is around...