Well, in all fairness, while I still think these so-called testicle prints are a load of horse manure, small testicles are possibly a sign that our hypothetical species is not engaging in sperm competition against other males. (Forgive me if I'm rehashing ground other people know far better than I do.) Some types of squirrel, for example, have extremely large testicles in proportion to their size as part of the accellerating "sperm war" because, since the females will mate with a number of males, competition takes place at the small 'n wiggly level. There's any number of interested methods, from massive overproduction of sperm (common to some primates) which compete through sheer force of numbers, to weirdly shaped "bottle brush" penises that dredge out some of the competition, to chemical warfare like spermicidal chasers and vaginal plugs.
So one could say, in a very broad sort of sense, that if these WERE testicle prints (I think this whole cast is suspect and absurd) and they were small in proportion to the body size, it would indicate that our hypothetical Bigfoot does not have the enlarged testicles of a species engaging in some types of sperm competition.
However.
It's not at all a direct one-to-one ratio that small testes = monogamy, and for anyone to claim that this proved Bigfoot's monogamy would be jumping the gun quite a ways. Gorillas, for example, have small testes and are still not classically monogamous--males keep harems, and defend them physically, and other males do steal females relatively often. There are lots of ways to compete that don't require your 'nads. And there's even a whole wild arsenal of neat stuff you can do to kill off your competitor's sperm without requiring testicles the size of cantelope, as mentioned above, so we can't even say with any authority that Bigfoot couldn't be engaging in sperm competition of some sort. Without a body, and without a lengthy period spent observing Bigfoot interaction and mating, there are serious limits. You just can't wring too many facts out of a dent in the mud.
All we could say is that the size of the testicles--assuming we had conclusive proof of this size, and not one "No-really-guys-it's-a-butt-print"--would indicate that Bigfoot is probably not engaging in the sort of sheer numbers sperm competition found in some other primates like chimps.
(Much of this is from memory, I freely confess, and may be old, but if anybody wants documentation, just yell, and I'll dredge some up. Bear in mind that my degree in anthropology was cultural, and merely a BA, and I've been an artist instead of a researcher for years, so caveat emptor. Still, the day I pass up a chance to talk about those pervy ground squirrels...)