Convincing a slam she's the only one for you, as you dig through an economy-size box of condoms. TFM.

This is funny because it is a common strategy among fraternity men to court women they are attempting to bed with flattering them with suggestions of a serious romantic relationship, since women tend to be more averse to casual sexual relationships than men due to social pressures and biological instinct and are more likely to engage in sex with a man they believe is only interested in them. The poster is pointing out the fallacious nature of these proclamations of monogamy by making one himself literally at the exact moment he is pulling a latex prophylactic out of a bulk box, which one would presumably only have purchased to facilitate safe sex with his multiple, regular casual sex partners.