As someone who has 3 mates married to women half their age (literally), i can shed some light on this subject.
The older (and uglier they are), the less chance there is of them straying. Many a young women has had a gutful of chauvinistic young guys playing around behind their backs, and so enjoys feeling more secure in an age-gap relationship.
But it's important to note that older guys are not elderly, meaning 70's or 80's with perhaps one foot in the grave. Now that is a bit sick depending how 'young' the young partner is of course.
I actually did a hot seat interview with a mate of mine a few months back on this very subject (sorry, can't post the location due to Hub rules), and he's 32 years older than his wife, and what's more they're just about to have their first baby.