Fleming discussed it in MR. All the 00s typically engaged in flings with married women.
Not necessarily all of them, though they do, according to
Moonraker, chapter one, make "determined assaults" on the virtue of their secretary, Loelia Ponsonby.
Actually, I think Craig's line about romances with married women keeping things simple is extrapolated from a couple of references in the books. In
Casino Royale, chapter 22, Bond reflects on his dissatisfaction with the progression of the typical love affair, which leads to a "mess of disentanglement." In
Moonraker, chapter one, he's "making love with rather cold passion, to one of three similarly disposed married women," which means, I suspect, that the women in question are neglected by their husbands and often alone. Although Fleming doesn't say so, Bond might think that such a relationship "keeps things simple," because all concerned would realize that the relationship isn't going anywhere, apart from a few shared moments of passion, and, unlike the dreary affairs described in
Casino Royale, can be broken off at any time without guilt, recrimination, or regret.
As Trident reminds us, however, real life isn't like that. Where human emotions are concerned, relationships that start out as simple have this annoying way of turning very complex indeed.
