twins
Explanation
This comic features two identical twin men approaching a woman at what appears to be a cocktail party, suggestively asking if she'd be interested in a "menage a trois... with twins." The woman declines, explaining: "Sorry, no. From a gene perspective, that's just regular sex."
The joke applies evolutionary biology to deflate a sexual proposition. Since identical twins share the same DNA, from a genetic standpoint, having sex with both of them is reproductively equivalent to having sex with just one -- there's no additional genetic diversity gained. The woman's response treats the situation with cold scientific rationality, reducing what the twins frame as an exotic proposition to something genetically mundane. It's a classic SMBC move of using scientific literalism to undercut a social situation in the most deflating way possible.