Yes, a "perceived" investment. As adept as women are at wearing gamefaces and keeping secrets, as well as the fact that the boyfriend is the only person out of the loop while the side man knows and enjoys his place, there's nothing REALLY wrong with making sure you aren't getting played, sick or pinned with a baby that isn't yours.
Yes checking your girls phone routinely while clutching yours tightly is hypocritical, but I don't know where
@TRUEST and other phone-check advocates stand on the availability of their phones.
You can't control infidelity but you can find out it's occurring and move accordingly while you're least affected.