Why less attractive males have to mate-guard more.

from The Myth of Monogamy by David P. Barash page 35 nonfiction ~1 min read

As a general rule, since the females of high-quality males are less likely to engage in EPCs, high-quality males have less need to guard. Older, more attractive males thus have a double advantage over their younger coun­terparts: Not only are they evidently appealing as sexual partners to already-mated females, but because they are so desirable, their own females are less likely to engage in EPCs. So these males have little need to spend time and effort mate-guarding and are therefore freed up to seek EPCs. The general pattern is concisely described in the title of one research article: "Unattractive Males Guard Their Mates More Closely." Several studies have confirmed that poor-quality males are generally more concerned with mate-guarding than are their high-quality counterparts, and for good reason, since females whose mates are less desirable are more inclined to seek EPCs. Males in poor condition are the Avis of Aves: They try harder.

—David P. Barash, The Myth of Monogamy, p. 35